According to McClatchy Newspaper report, the relationship began with his first arrest back in Jun 1988, when customs agents caught Headley for smuggling heroin from Pakistan to US.
"He wasn't tough to crack. Before the day was out, Daood Gilani alias David Headley decided to save his own skin, agreeing to betray his drug-dealing partners by helping US drug agents set up a sting," McClatchy report was quoted as saying.
Headley was so helpful as a DEA informant in the late 1990s on heroin imported from Pakistan that prosecutors made a rare move. They ended his probation years early, allowing him to travel freely.
He was briefly married to a Philadelphia woman in the 1980s.
He later became a heroin addict, and twice was caught smuggling the drug into the country by the DEA, in 1988 in Frankfurt, Germany, and in 1997 in New York.
Federal law enforcement officials in New York realised that Headley knew a great deal about the heroin trade between the US and Pakistan and was willing to cooperate, McClatchy report added.