The FBI said Headley attended militant training camps in Pakistan
A US citizen has pleaded guilty to scouting targets for the 2008 attacks on the Indian city of Mumbai.
David Headley, at his trial in Chicago, also admitted plotting to attack a Danish newspaper that published a cartoon many Muslims deemed offensive.
Headley, 49, had denied the charges but changed his plea to avoid the death penalty or extradition to India, Pakistan or Denmark.
The attacks on Mumbai in November 2008 left more than 170 people dead.
In changing his plea, Headley admitted 12 counts of conspiracy.
Prosecutors will therefore ask for a lesser sentence, but US District Court Judge Harry Leinenweber said Headley could still be jailed for life.
Born in 1960 in the US to a Pakistani father and an American mother
Spent much of his childhood in Pakistan
Attended a military boarding school in Islamabad
Dropped out of school at 17
Joined his mother in the US after she divorced his father
Changed his name to David Headley in 2006
Made five extended trips to Mumbai between 2006 and 2008
FBI says he took pictures and made videotapes of various targets, including those attacked in 2008
Arrested in Chicago on 3 October 2009 as he was about to travel to Pakistan
US Attorney General Eric Holder said in a statement: "Not only has the criminal justice system achieved a guilty plea in this case, but David Headley is now providing us valuable intelligence about terrorist activities."
Prosecutors said Headley, a Pakistani-American, had made several surveillance trips to India and Denmark.
According to court documents, he passed on information to his contacts with the Pakistan-based Islamic militant group, Lashkar-e-Taiba.
The group has been blamed for organising the Mumbai attacks.
Headley was arrested by FBI agents in Chicago in October while trying to board a plane for Philadelphia.
He is alleged to have told prosecutors that he had been working with Lashkar-e-Taiba since 2002.
He was first charged with plotting to attack the Danish newspaper Jyllands-Posten after they angered Muslims by publishing cartoons of the Prophet Mohammed.
Headley changed his name from Daood Gilani in 2006 after he was told by members of Lashkar-e-Taiba that he would be travelling to India to carry out surveillance duties for the group, prosecutors said.