Born in Washington D.C, he became a U.S. Drug Enforcement Administration (DEA) informant as part of his plea deal following multiple heroin related offenses, including attempting to smuggle narcotics into the U.S. from Pakistan.

The following year, he performed a similar mission in Copenhagen, Denmark to help plot an attack against the Danish newspaper Jyllands-Posten, which had published cartoons of Muhammad.

David Coleman Headley was born Daood Sayed Gilani on June 30, 1960, in Washington, D.C., to a Pakistani father and an American mother.

Danyal became the spokesman for Pakistani Prime Minister Yousaf Raza Gillani and currently serves as Pakistan's press attaché in Beijing.

[12] Headley went on to attend the elite Cadet College Hasan Abdal, a boys' international military prep school, while there, he befriended co-conspirator Tahawwur Hussain Rana.

In January 1998, the agency sent him to Pakistan to dispel suspicions amongst his partners about his prior absence, and to gain intelligence on the country's heroin trafficking networks.

A mentally impaired Pakistani immigrant, Ikram Haq, was found to have been tricked into making a drug deal by Headley, and was subsequently acquitted on the grounds of entrapment when brought to trial.

[23] A day after the attacks on September 11, 2001, Headley's DEA handlers tasked him with collecting counter-intelligence on terrorists through his sources in the drug trade.

[26] On November 16, 2001, six weeks after his interrogation, Leader and Assistant U.S. Attorney Loan Hong made a joint application to Judge Amon asking for Headley's supervised release to be terminated three years early.

[24] Leader has claimed that the DEA was involved in the drive to end Headley's probation, which would have kept him from traveling to Pakistan to continue his intelligence work on terrorists.

That summer, Serrill Headley, who by then had moved to the town of Oxford, Pennsylvania with her brother, confided to friends that her son had become a religious fanatic and had been to terrorist training camps.

During her immigration interview she made reference to Headley's radicalization and terrorist training; his anti-Semitic and anti-Hindu prejudices; and his praise for suicide bombers.

Even though Pennsylvania law requires a background check for name changes, state officials apparently did not uncover Headley's previous drug convictions.

In February, Headley was again detained by border inspectors at New York's John F. Kennedy International Airport after returning from Pakistan, and was again released.

Headley stayed at the Taj Palace Hotel—identified by Iqbal and Mir as their main target—and surveyed the building using his ISI training, shooting hours of video during in-house tours.

[30] Headley also took boat tours to look for places where the attackers could reach the city through the waterfront; he found a landing location at a fishermen's slum in the Colaba area of southern Mumbai, where he gathered GPS coordinates.

Elsewhere during his trip, Headley collected video footage of Copenhagen, including the offices of Jyllands-Posten; looked into leasing an apartment that could be used by LeT's attack team; and inquired about getting a job as a secretary.

Syed became Headley's new handler and introduced him to Ilyas Kashmiri, leader of Harkat-ul-Jihad al-Islami, an Islamist organization active in Pakistan, India, and Bangladesh.

[31][32][33] Kashmiri took over sponsorship of the Denmark plot and made changes to the plan, using the Mumbai attacks as inspiration; he wanted terrorists to storm the Jyllands-Posten offices, have its staff be taken hostage and executed, and then have their severed heads thrown out of the windows of the newsroom in an international media spectacle.

When Headley arrived in Copenhagen by train on July 31, he shot video of a Royal Danish Army barracks and approached drug dealers about acquiring guns.

On October 9, Headley was arrested at Chicago's O'Hare International Airport while he was attempting to travel to Pakistan to deliver the footage he collected in Denmark.

Supervised by federal agents, he helped set up a trap against a militant in Germany and attempted to lure Sajid Mir out of Pakistan.

[41] Indian investigators were surprised at how easily Headley had obtained a visa to enter India, a process that is extremely difficult for Pakistani nationals and Pakistani-origin individuals.

Some Indian analysts have speculated that David Headley was a double agent for the Central Intelligence Agency that had infiltrated LeT,[47] an accusation denied by the CIA.

[citation needed] Saeed, with the help of Ilyas Kashmiri, drafted Headley for the plan to attack the Danish newspaper Jylland Posten, which had published cartoons of Muhammad considered controversial by Muslims.

[56] On May 31, 2011, however, Headley contradicted his previous statements, and testified that the Inter-Services Intelligence (ISI) leadership was not involved in planning the 2008 Mumbai attacks.

In relation to the 2010 Pune blast at the German bakery that injured at least 53 people and killed 18, of whom 6 were foreigners,[57] Indian Home Secretary, G. K. Pillai[58] and the Hindustan Times[59] referred to Headley.

[63] In July 2015, it was reported that the Mumbai police were seeking to take a deposition of Headley by video conference to provide evidence against Zabiuddin Ansari.

On February 9, 2016, it was reported that Headley had confessed to a Mumbai court about LeT and Inter-Services Intelligence having penetrated into the ranks of the Indian Army, to work as spies.

[66] On July 24, 2018, it was reported that Headley was seriously injured after being attacked in prison, and subsequently admitted to the critical care unit of Evanston Hospital of the NorthShore University HealthSystem.