Chinese state media demanded severe punishment on Thursday for those behind what China has said is a holy war aimed at Beijing, which it has blamed on Islamist militants from the restive Xinjiang region.
The exiled leader of Xinjiang's Muslim Uighur minority called for an independent probe into a crash on Monday when an SUV burst into flames after being driven into a crowd in Tiananmen Square, one of China's most closely guarded areas. The three occupants of the vehicle and two bystanders were killed, and dozens were injured.
U.S.-based Rebiya Kadeer said she did not believe any kind of organised extremist Islamic movement was operating in Xinjiang, a view shared by rights groups and some experts.
"It is almost impossible for Uighurs to organise because of China's stringent controls and attacks," she said in an interview.
But police said Monday's incident was a carefully planned and organised "terrorist attack" carried out by people from Xinjiang. They announced they had apprehended five accomplices in Beijing who they said were Islamist militants planning a holy war. Their names suggest they are Uighurs.
Such an attack is a crime against humanity, the city's official Beijing Daily said in a commentary. The government should spare no effort to ensure Beijing's safety, it added.
"Violent terrorist crime is the shared enemy of all humanity, the shared enemy of all ethnic groups in the country, and it must be severely punished under the law," said the commentary, which was also carried by the website of the ruling Communisty Party's People's Daily website.
"Maintaining the capital's security and stability is a responsibility of utmost importance."
The English-language China Daily said the perpetrators will "go down in history as murderers not heroes".
Xinjiang, in China's west, has been beset by violence, blamed by China on Uighur separatists and extremists. Many Uighurs in the region, which borders Central Asian nations that were part of the former Soviet Union as well as Afghanistan and Pakistan, chaff at Chinese controls on their religion, culture and language, although the government says they enjoy widespread freedoms.
Kadeer, president of the Munich-based World Uyghur Congress, the main exiled Uighur organisation, cautioned against believing China's account of the incident.
"Chinese claims simply cannot be accepted as facts without an independent and international investigation of what took place in Beijing on Monday," Kadeer said.
China, which calls Kadeer an "anti-Chinese splittist", will almost certainly ignore her call for an international investigation.
Authorities have tightened security in energy-rich Xinjiang, with added police presence on the streets. Armed police prevented Reuters reporters from entering Lukqun town, where one of the detained suspects is from, sending them back to the nearby city of Turpan.
Security has also been tightened in Beijing with extra police at the airport and on the streets. Some residents expressed fear about how unrest in Xinjiang had apparently made its way to the capital.
"Actually I feel very afraid because ... I often go over to Tiananmen. I thought something like this would be so far off from happening to us here, but for this to suddenly happen so close to me ... I just feel worried and scared," said Zhang Xiaoyan, 26, who works in financial services.
Kadeer said Uighurs may or may not have been responsible for the attack on Monday.
"It is difficult to tell at the moment, given the strict control of information by the Chinese government on this tragic incident," she said.
"If the Uighurs did it, I believe they did it out of desperation because there is no channel for the Uighur people to seek redress for any kind of injustice they had suffered under Chinese rule."
Her comments were made in written replies to Reuters questions, translated from the Uighur language by an aide.
Kadeer is a former Chinese political prisoner who was accused of leaking state secrets in 1999. She left China on medical parole and settled near Washington with her husband and part of her family in 2005. The 66-year-old mother of 11 was previously a celebrated millionaire who advised China's parliament.
Kadeer said she feared the Tiananmen Square attack would join a long list of incidents that China uses "to justify its heavy-handed repression" in her native region.