Zarein Ahmedzay had earlier entered a plea of not guilty
A New York man has pleaded guilty to terror charges in connection with a plot to set off suicide bombs in the city's subway system.
Zarein Ahmedzay admitted conspiracy to use weapons of mass destruction and providing support to al-Qaeda, in a plea entered in a court in New York.
He was a classmate of Afghan immigrant Najibullah Zazi, who pleaded guilty to his role in the plot last month.
US officials have called it one of the most serious threats since 9/11.
'Meeting with leaders'
Ahmedzay, 25, originally entered a not guilty plea after his arrest in January.
He was picked up with another classmate, Adis Medunjanin, who is facing similar charges.
Prosecutors say the three were planning an attack on city subway lines last September under the direction of al-Qaeda that would have been similar to the 2005 attacks on public transit in London, which killed more than 50 people.
Ahmedzay pleaded guilty at Brooklyn federal court.
Prosecutors have said Ahmedzay, Zazi and Mr Medunjanin all travelled to Pakistan in 2008 to look for terrorism training.
At the Brooklyn hearing, Assistant US Attorney Geoffrey Knox said the plotters had met two senior al-Qaeda operatives who ordered them to carry out the suicide bombings.
Mr Knox identified the leaders as Saleh al-Somali and Rashid Rauf, both of whom were killed in Pakistan over the past year.
Zazi is a Colorado airport van driver who had pleaded guilty to testing bomb-making equipment in Denver and then travelling to New York to take part in the plot.
The scheme was said to have been uncovered when police stopped Zazi's car as he entered the city.
Ahmedzay and Zazi are of Afghan origin and Mr Medunjanin is originally from Bosnia. Ahmedzay and Mr Medunjanin are US citizens.