US officials received at least five warnings that an American man 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 David Coleman Headley or place him on any watchlist, the report said, citing a review underway for US Director of National Intelligence (DNI) James Clapper.

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 it said prompted the DNI review.

Despite the warnings, Headley continued to move freely, traveling 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.

That last tip, which had not been previously reported, involved one of Headley's ex-wives, who told US officials overseas that she suspected he was involved in a 2007 bombing that killed 68 people in India and was also blamed on Lashkar-e-Taiba.

"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."

In December 2007, Headley's Moroccan wife even approached the US embassy in Pakistan to give stark warnings to US State Department agents with Immigration and Customs Enforcement and the Bureau of Diplomatic Security.

She told them her husband was "looking to participate in jihad against the US" and discussed suicide bombings and his training with militant groups, ProPublica said, citing US officials.

Headley, the son of a former Pakistani diplomat and a white American woman, is being held in the United States.

He has confessed to plotting the Mumbai attacks and in exchange for pleading guilty, US prosecutors agreed he would not face extradition to India or the death penalty.

US President Barack Obama meanwhile met on Saturday with families of victims and survivors at the luxury Taj Mahal Palace hotel, which was the focus of the wave of strikes in the 2008 assault.