This picture provided by J.P. Karas shows Northwest Airlines Flight 253 on the runway after arriving at Detroit Metropolitan Airport from Amsterdam on Friday, December 25, 2009. [Agencies]

WASHINGTON: A Northwest Airlines passenger from Nigeria, who said he was acting on al-Qaida's instructions, tried to blow up the plane Friday as it was landing in Detroit, law enforcement and national security officials said.

Passengers subdued the man and may have prevented him from detonating the explosives, the officials said.

"We believe this was an attempted act of terrorism," a White House official said.

Federal officials imposed stricter screening measures after the incident.

There was nothing out of the ordinary until the flight was on final approach to Detroit, said Federal Aviation Administration spokeswoman Elizabeth Isham Cory. That is when the pilot declared an emergency and landed without incident shortly thereafter, Cory said in an e-mail message. The plane landed at 11:51 am EST.

One of the US intelligence officials said the explosive device was a mix of powder and liquid. It failed when the passenger tried to detonate it.

The passenger was being questioned Friday evening. An intelligence source said the Nigerian passenger was being held and treated in an Ann Arbor, Mich., hospital.

All the sources spoke on condition of anonymity because the investigation was continuing.

One law enforcement source said the man claimed to have been instructed by al-Qaida to detonate the plane over US soil.

The official said an official determination of a terrorist act would have to come from the attorney general.

The official added that additional security measures are being taken without raising the airline threat level.

The official declined to describe what additional measures law enforcement was taking.

The White House was coordinating briefings for the president through the Homeland Security Department, the Transportation Security Administration and the FBI.

A law enforcement source said the explosives may have been strapped to the man's body but investigators weren't immediately certain, partly because of the struggle with other passengers.