Headley's attorney John T. Theis said he would "continue to look at this and see what the evidence is," but declined to comment further.
Authorities in Washington said Headley has cooperated with investigators in both the Danish and Indian plots since his arrest.
A retired major in the Pakistani military, Abdur Rehman Hashim Syed, was charged with conspiring to attack the Danish newspaper and its employees.
Pakistan's army has confirmed it has a retired major in custody in connection with the U.S. terror investigation. Military spokesman Maj. Gen. Athar Abbas did not say when the arrest was made or reveal the identity of the man but said last week that the major was being questioned over alleged links to Headley and Rana.
Headley, 48, an American citizen formerly named Daood Gilani, and Chicago businessman Tahawwur Rana, 48, a Canadian national, were charged in October with plotting to attack the Jyllands Posten newspaper in Denmark. The newspaper had published 12 cartoons in 2005 that depicted the Prophet Muhammad and set off protests in parts of the Islamic world.
U.S. Attorney Patrick J. Fitzgerald said the "investigation remains active."
Federal prosecutors said at the time of his arrest that Headley admitted his role in a plot against the newspaper and that he had received training from Lashkar-e-Taiba -- a group that specializes in violence against India.
The charges filed in U.S. District Court on Monday said Headley had attended Lashkar-e-Taiba training camps in Pakistan earlier this decade and conspired with members of the group to launch terrorist attacks in India.
Indian External Affairs Ministry spokesman Vishnu Prakash did not immediately respond to messages late Monday seeking comment.
Prosecutors said Headley changed his name from Daood Gilani in 2006 so that he could pass in India for an American who was neither Muslim nor Pakistani. They said he later made five extended trips to Mumbai from September 2006 through July 2008, taking pictures of various targets.
Among the targets he allegedly scouted were the Taj Mahal and Oberoi hotels, the Leopold Cafe, the Nariman House and the Chhatrapati Shivaji Terminus train station -- each of which was attacked with guns, grenades and other explosives in the November 2008 attacks.
Lashkar-e-Taiba -- the Army of Good -- is a group that has been outlawed in Pakistan and designated by the United States as a foreign terrorist organization. But experts say it has ties to individuals in Pakistan's military, which has been feuding with India for decades over the territory of Kashmir.
The U.S. attorney's office said Lashkar-e-Taiba tasked Headley in late 2005 with gathering surveillance on Mumbai targets. It said he traveled to Chicago in June 2006 and advised a person identified in the charges only as Individual A of the plan. He then allegedly got Individual A's approval of a plan to open an office of First World Immigration Services in Mumbai as cover for his work.
Rana has operated First World Immigration Services.
A two-count complaint against Abdur Rehman was filed under seal Oct. 20. It says he coordinated surveillance of the Danish newspaper and participated in planning the attack there along with Lashkar-e-Taiba and Ilyas Kashmiri, who was described as a leader of the terrorist group Harakat-ul Jihad Islami.
Headley visited Pakistan in January and at that time, authorities say, Abdur Rehman took him to the Federally Administered Tribal Areas along that country's western edge where a number of terrorist groups have allegedly found refuge. The purpose of the trip was to meet with Kashmiri and solicit his help in launching the attack against the Danish paper, the charges say.
A search of Headley's luggage at the time of his arrest turned up a list of phone numbers including one allegedly used to contact Abdur Rehman.
Pakistan's Interior Minister Rehman Malik, asked earlier Monday about Headley, said the government was willing to cooperate with U.S. authorities.
"If he has committed something, he should be punished as per American law," Malik said. "If he had any relations in Pakistan, and whatever information they will give us, the information we will receive bilaterally, or internationally or through Interpol, whatever help we could give, we will certainly do."