US officials received at least five warnings that an American man, David Coleman Headley, who became a key figure in the 2008 Mumbai attacks was training or working with Pakistani extremists, The Washington Post said on Saturday.
Despite the warnings to US intelligence agencies building up over seven years, officials did not move to question Headley or place him on any watchlist, the report said, citing a review underway for the US director of national intelligence (DNI).
The report was co-published by the Post and ProPublica, an investigative journalism group that reported the story.
Last month, ProPublica revealed that one of Headley's wives had warned FBI agents in August 2005 that her husband had undergone intensive training with Lashkar-e-Taiba and "was an active militant" with the radical extremist group, in a report the newspaper said prompted the DNI review.
Despite the warnings, Headley continued to move freely, travelling to Pakistan, India, Dubai and Europe in 2006, gathering information and material that made possible the attacks by the Pakistani militants on Mumbai, which left 166 people dead and more than 300 others injured.
US intelligence officials got their first tip as far back as early October 2001, just weeks after the September 11 attacks on New York and Washington, when a former girlfriend told agents that Headley supported Pakistani extremists and voiced readiness to fight in the region for the group's cause.
After interviews, however, including with Headley's wealthy mother Serrill, the investigation was closed because authorities saw no danger. There were further tips to US intelligence agencies in July 2002, in Philadelphia, and then in 2005, 2007 and just seven months before the attacks in 2008.
"It's a black eye," a senior anti-terror official told ProPublica, noting officials failed to connect the dots between the separate tips.
"The problem is the information system. New York didn't know about Philadelphia. Islamabad didn't know about Philadelphia or New York."
Headley, the son of a former Pakistani diplomat and a white American woman, is being held in the United States.