Indian officials have long suspected that Pakistan was somehow connected to attacks in Mumbai in 2008 that killed 166 people and wounded hundreds more. The New York Times reported this week that a previously unidentified player is a recently retired high-ranking Pakistani military officer.
The report is expected to put further strain on Pakistan's relations with India, but also with the United States.
"It's an excellent illustration of how deep the Islamist sympathies run in the Pakistani military," said Professor Tristan Mabry, an expert on India-Pakistan relations with the U.S. Naval Postgraduate School in Monterey, Calif.
Prof. Mabry estimates that the United States gives Pakistan about $1.5-billion in economic aid each year plus $1.5-billion in military funding. This infuriates India, he said, which sees itself as a victim of Pakistani-supported terrorism.
An FBI affidavit unsealed this month makes repeat references to someone it identifies only as "Individual B." Citing sources, The New York Times reported that the individual is an ex-military officer, but did not give a name.
The affidavit was filed after the arrests of Tahawwur Rana, a Pakistani-born Canadian who has lived the past 10 years in Chicago, and his boyhood friend David Headley, who was known as Daood Gilani until three years ago. None of the allegations against the two men has been proven in court.
Based on intercepts of phone calls and e-mail exchanges - primarily between Mr. Rana, Mr. Headley, the ex-Pakistani military official and two men listed by the U.S. as senior terrorism figures - the FBI arrested Mr. Rana and Mr. Headley last month under suspicion of planning a terrorist attack.
The FBI's case focused primarily on claims the two men planned to attack the offices of the Denmark newspaper Jyllands-Posten over cartoons of the prophet Mohammed that infuriated many Muslims. The FBI's affidavit notes, however, that the plot appeared to shift this fall to an attack in India.
New information about Mr. Headley surfaced this week in a profile by the Philadelphia Examiner. The paper had interviewed Mr. Headley's mother, Serrill Headley, in 1974.
According to that 35-year-old interview, Ms. Headley, who grew up in the Philadelphia suburb of Bryn Mawr, said she married a Pakistani diplomat and lived in Pakistan for 10 years, but the marriage fell apart and she lost custody of her children.
In 1977, she brought her son to Philadelphia, but she said he had a hard time accepting that she ran a bar because of his Muslim upbringing.
In recent years, Mr. Headley and Mr. Rana both posted regular messages on a Yahoo message group for former students of a military school in Hasan Abdal, Pakistan, according to the FBI.
In one 2008 post, Mr. Headley wrote that Islam should not be mocked, and that he was "disposed toward violence" against the Danish cartoonists and authors Sherry Jones and Irshad Manji, a Canadian.