A review being conducted for the director of national intelligence has found at least five cases in which U.S. agencies were warned that David Coleman Headley, who became a central figure in the 2008 attacks on Mumbai, was training or working with Pakistani militants.

In the seven years in which leads accumulated, Headley was not questioned or placed on a terror watch list, officials say.

The review, which is not complete, has found that allegations about the Pakistani American businessman's extremist ties began as early as 2001 and were more numerous and specific than previously disclosed, officials said. They described to ProPublica the results of internal inquiries being conducted by the FBI, the CIA and other agencies.

In a previously unreported tip just seven months before the Mumbai attacks, one of Headley's ex-wives told U.S. officials overseas that she suspected he was linked to a 2007 bombing in India that killed 68 people and has been blamed on the Lashkar-e-Taiba militant group. She also warned that Headley was on a "special mission," according to a senior anti-terror official.

Since the Sept. 11 attacks, the government has invested billions of dollars in new threat-detection systems. But the Headley case suggests that flawed information-sharing, an overwhelming flow of raw intelligence and a lack of focus on Lashkar kept investigators from identifying the threat posed by an American terrorist.

"It's a black eye," said the senior anti-terror official, who spoke on condition of anonymity. "The problem is the information system. New York didn't know about Philadelphia. Islamabad didn't know about Philadelphia or New York."

The DNI launched a review of the Headley case after ProPublica and The Washington Post reported last month that federal investigators in New York City looked into a 2005 tip from Headley's wife. The New York Times then reported a tip from another of his wives in Pakistan in 2007.

The review has found four additional warnings, in 2001, 2002, April 2008 and December 2008 - a month after Lashkar killed 166 people in Mumbai, six of them Americans. Headley, 50, was not arrested until October 2009. He has pleaded guilty to terrorism-related charges and is cooperating.

DNI spokeswoman Jamie Smith said she could not comment on the findings.

"Reviews of this nature are not uncommon and an important part of improving existing processes," Smith said.

Federal officials point out that the first two tips surfaced after the Sept. 11 attacks, when agencies struggled to modernize databases.

The review is expected to address another question: whether Headley's work as a U.S. informant affected investigations of him. Headley spied on Pakistani drug traffickers for the DEA starting in the late 1990s, though officials say the DEA cut ties with him "well before" Mumbai.