The United Nations investigators assembled in the departure hall of Sanaa's airport were preparing to leave with precious evidence: laptops and external drives collected from the staff of the World Health Organization.
These computers, they believed, contained proof of corruption and fraud within the UN agency's office in Yemen.
But before they could board their flight out, armed militiamen from the Houthi rebels ruling northern Yemen marched into the hall and confiscated the computers, according to six former and current aid officials.
The stunned investigators were left unharmed, but flew out without the telltale devices.
The Houthis had been tipped off by a WHO staffer with connections to the rebel movement who feared her theft of aid funding would be uncovered, according to the six former and current officials who spoke on condition of anonymity because the seizure of the computers had not previously been made public.
The October 2018 scene at the Sanaa airport is another episode in the continuing struggle over corruption that has diverted donated food, medicine, fuel and money from desperate Yemenis amid their country's five-year civil war.
More than a dozen UN aid workers deployed to deal with the wartime humanitarian crisis have been accused of joining with combatants on all sides to enrich themselves from the billions of dollars in donated aid flowing into the country, according to individuals with knowledge of internal UN investigations and confidential documents reviewed by The Associated Press.
The AP obtained UN investigative documents, and interviewed eight aid workers and former government officials.
The upshot: WHO internal auditors are investigating allegations that unqualified people were placed in high-paying jobs, millions of dollars were deposited in staffers' personal bank accounts, dozens of suspicious contracts were approved without the proper paperwork, and tons of donated medicine and fuel went missing.
A second probe by another UN agency, UNICEF, focuses on a staffer who allowed a Houthi rebel leader to travel in agency vehicles, shielding him from potential airstrikes by the Saudi-led coalition. The individuals who spoke to AP about the investigations did so on condition of anonymity for fear of reprisals.
Yemeni activists said the actions by the UN agencies are welcome but fall short of the kind of investigation needed to track the millions of dollars in supplies and money from aid programs that have gone missing or been diverted to the coffers of local officials on both sides of the conflict since the start of the civil war.
Over the past three months, the activists have been pushing for aid transparency in an online campaign called "Where Is The Money?'' They demand that UN and international agencies provide financial reports on how the hundreds of millions of dollars pouring into Yemen since 2015 have been spent.
Last year, the agency said international donors pledged $2 billion for humanitarian efforts in Yemen.
The UN has responded with an online campaign of its own called "Check Our Results,'' showing programs implemented in Yemen. The campaign does not provide detailed financial reports on how aid money is spent.
"We see big numbers, billions of dollars reaching Yemen, and we don't know where they go,'' Feda Yahia, a "Where is the Money?'' activist, said in a video for the campaign.
The WHO probe of its Yemen office began in November with allegations of financial mismanagement against Nevio Zagaria, an Italian doctor, who was chief of the agency's Sanaa office from 2016 until September 2018, according to three individuals with direct knowledge of the investigation.
The only public announcement of the probe came in a sentence buried in the 37 pages of the internal auditor's 2018 annual report of activities worldwide. The report did not mention Zagaria by name.
The report, released May 1, found that financial and administrative controls in the Yemen office were ``unsatisfactory'' _ its lowest rating _ and noted hiring irregularities, no-competition contracts and lack of monitoring over procurement.
WHO spokesman Tarik Jasarevic confirmed to the AP that the investigation is underway. He said Zagaria retired in September 2018, but he would not confirm or deny that Zagaria specifically was under investigation.
"The Office of Internal Oversight Services is currently investigating all concerns raised,'' he said. "We must respect the confidentiality of this process and are unable go into details on specific concerns.''
Zagaria did not respond to emailed questions from the AP.
Zagaria, a 20-year WHO employee, arrived in Yemen in December 2016, after a four-year stint in the Philippines. He had earned widespread acclaim for his handling of the agency's response to Typhoon Haiyan in November 2013.
Because of his work during the typhoon, Zagaria seemed the perfect person to lead the agency's humanitarian efforts in Yemen, a massive operation, providing support for more than 1,700 hospitals and health centers around the country.
But from the beginning, six current and former workers said, the WHO's Yemen office under Zagaria was riddled with corruption and nepotism.
Zagaria brought in junior staffers who worked with him in the Philippines and promoted them to high-salary posts that they were not qualified for, three individuals said.
Two of them _ a Filipino university student and a former intern _ were given senior posts, but their only role was to take care of Zagaria's dog, two of the officials said.
"Incompetent staff with heavy salaries'' undermined the quality of work and monitoring of projects and created ``many loopholes for corruption,'' a former aid official said.
Zagaria also allegedly approved suspicious contracts signed by staffers with no competitive bidding or documentation for the spending, according to internal documents reviewed by the AP.
The documents show that local firms contracted to provide services at WHO's Aden office were later found to have hired WHO staffers' friends and family and overcharged for services. The owner of one firm was seen handing cash to one staffer, the documents show _ an apparent kickback.
Four people aware of the activities in the office said a WHO staffer named Tamima al-Ghuly was the one who notified the Houthis that the investigators were leaving with the laptops. They said she had been fabricating payrolls, adding ghost employees and collecting their salaries or taking cash to hire people. Among those she put on the payroll was her husband, a member of a leading Houthi family, they said.
Al-Ghuly has since been suspended, but remains a WHO employee, according to an individual with direct knowledge of the incident. She did not respond to AP's attempts to reach her.
Under Zagaria, aid funds meant to be spent during emergencies were also used with little accountability or monitoring, according to internal documents.
Under WHO rules, aid money can be transferred directly into the accounts of staffers, a measure meant to speed up the purchasing of goods and services in a crisis. The WHO says the arrangement is needed to keep operations going in remote areas because Yemen's banking sector is not fully functioning.
Because they are supposed to be restricted to emergencies, there is no requirement that spending of these direct transfers be itemized. Zagaria approved direct transfers of cash worth a total of $1 million for certain staffers, according to internal documents. But in many cases it was unclear how they spent the money.
Omar Zein, a deputy head of the agency's Aden branch who worked under Zagaria, received several hundred thousand dollars in aid money to his personal account, according to interviews with officials and internal documents. But Zein could not explain what happened to more than half of the money, internal documents show.
Four individuals with direct knowledge of the aid operations in southern Yemen said that even as Zein held his WHO position, he also served as an official adviser to the health minister in the Aden-based government and ran his own private nonprofit that had a $1.3 million contract with the UN to run nutritional programs in the southern city of Mukalla. These arrangements created conflicts of interest, the individuals said.
UNICEF later refused to renew the contract with Zein's nonprofit after discovering the organization was fabricating reports and had no actual presence on the ground in the city of Mukalla, two individuals with knowledge of the programs said.
When contacted by the AP, Zein declined comment and said he had left his post at the health ministry.
Asked if he were under investigation for corruption, he said, ``The one who leaked this to you can provide you an answer.''
WHO isn't the only UN agency looking into allegations of wrongdoing by its staffers in Yemen.
According to three people with knowledge of the probe, UNICEF is investigating Khurram Javed, a Pakistani national who is suspected of letting a senior Houthi official use an agency vehicle.
That effectively gave the Houthi official protection from airstrikes by the Saudi-led coalition fighting the Houthis, since UNICEF clears its vehicles' movements with the coalition to ensure their safety. Officials say they fear the agency's vehicles could be targeted by airstrikes if coalition forces believe they are being used to shield Houthi rebels.
Javed was well known for his close ties to Houthi security agencies; he boasted that he used his connection to prevent UNICEF auditors from entering the country, a former co-worker and an aid official said. The Houthi rebels even put up a large billboard of him on a Sanaa street, thanking him for his services.
Javed could not be reached for comment. UNICEF officials confirmed that as part of an ongoing probe, an investigative team had traveled to Yemen to look into the allegations. They said Javed has been transferred to another office but did not disclose the location.
According to several people who spoke to the AP, close ties between UN staffers and local officials on both sides of the conflict are common.
A confidential report by a UN panel of experts on Yemen, obtained by the AP, said Houthi authorities constantly pressure aid agencies, forcing them to hire loyalists, intimidating them with threats to revoke visas and aiming to control their movements and project implementation.
Officials said it's unclear how many staffers may be aiding combatants. The officials said several incidents in recent years indicate the UN staffers may have been involved in the theft of aid supplies.
Internal UN reports from 2016 and 2017 obtained by the AP show several incidents where trucks carrying medical supplies were hijacked by Houthi rebels in the battleground province of Taiz.
The supplies were later given to Houthi fighters on the front lines fighting the Saudi-led coalition or sold in pharmacies in territory controlled by the rebel group.
An official who help draft the reports said it was ``obvious there were some individuals who were working with Houthis behind the scene because there was coordination on the movement of trucks.''
Another official said the UN's inability or unwillingness to address the alleged corruption in its aid programs harms the agency's efforts to help Yemenis affected by the war.
"This is scandalous to any agency and ruins the impartiality of UN,'' the aid official said.