Filipino doctor wanted over a foiled jihadist plot targeting New York's subway and Times Square previously treated pro-Islamic State (IS) group militants in the mountains of southern Philippines, the military said on Monday.
Russell Salic, 37, had links to the Maute group, which since May had been occupying parts of the Philippines' most important Islamic city of Marawi in a bid to establish an IS caliphate in Southeast Asia, Philippine authorities said.
"He was among those who were treating wounded members of the Maute group," military spokesman Colonel Edgard Arevalo told AFP.
Another military spokesman, Major-General Restituto Padilla, told reporters Salic had performed these tasks in visits to Maute training camps before the Marawi attack.
"That's why his nom de guerre or nickname, based on our information, was 'Doc' or 'Doctor,'" Padilla added.
Salic, who has been in Philippines custody since April, is wanted by US prosecutors after he and two other were indicted over a plan to conduct bombings and shootings in Times Square, New York's subway system and concert venues in the name of IS.
The attacks were planned for the Muslim holy month of Ramadan in 2016, US prosecutors said when they announced the charges on Friday.
One alleged plotter is already in US custody while the second is in Pakistani custody.
Salic is accused of wiring $423 to the US to help fund the plot, the American justice department said.
He and the suspect in Pakistan now face legal proceedings seeking their extradition to the United States.
The restive south of the mainly Catholic Philippines is home to a decades-old Muslim separatist insurgency and to extremist gangs that have declared allegiance to IS.
Among them is the Maute group, which had withstood a US-backed military campaign in Marawi including artillery and daily air strikes in fighting that had left 979 people dead and thousands displaced.
Salic is under investigation in Manila over his alleged involvement in the kidnapping and beheading of two sawmill workers in April 2016 in the southern rural town of Butig, which the military had blamed on the Maute group.
Four other sawmill workers were set free and told authorities they saw Salic in the Maute camp where they were detained, senior assistant state prosecutor Peter Ong told AFP.
"The complainants said they saw him in an adjacent room full of guns. He was cleaning guns," Ong said, quoting from the workers' depositions.
Salic had denied the allegation and said he was in another province then, according to Ong, who is handling the investigation and has yet to decide whether to charge Salic in court.