But a closer examination of the case, drawn from extensive reporting by ProPublica, shows that the government surveillance only caught up with Headley after the U.S. had been tipped by British intelligence. And even that victory came after seven years in which U.S. intelligence failed to stop Headley as he roamed the globe on missions for Islamic terror networks and Pakistan’s spy agency.

Supporters of the sweeping U.S. surveillance effort say it’s needed to build a haystack of information in which to find a needle that will stop a terrorist. In Headley’s case, however, it appears the U.S. was handed the needle first — and then deployed surveillance that led to the arrest and prosecution of Headley and other plotters.

As ProPublica has previously documented, Headley’s case shows an alarming litany of breakdowns in the U.S. counterterror system that allowed him to play a central role in the massacre of 166 people in Mumbai, among them six Americans.

A mysterious Pakistani-American businessman and ex-drug informant, Headley avoided arrest despite a half dozen warnings to federal agents about extremist activities from his family and associates in different locales. If those leads from human sources had been investigated more aggressively, authorities could have prevented the Mumbai attacks with little need for high-tech resources, critics say.

“The failure here is the failure to connect systems,” said a U.S. law enforcement official who worked on the case but is not cleared to discuss it publicly. “Everybody had information in their silos, and they didn’t share across the silos. Headley in my mind is not a successful interdiction of a terrorist. It’s not a great example of how the system should work.”

Officials from Clapper’s office reiterated this week that he was referring to the prevention of Headley’s follow-up role in a Mumbai-style attack against Denmark’s Jyllands-Posten newspaper, a prime target because it published cartoons of the Prophet Muhammad that many Muslims found offensive. To that extent, Clapper’s comment shed a bit of new light on this aspect of a labyrinthine case.

Separately today, NSA Director Gen. Keith Alexander told a Senate committee that surveillance conducted by his agency helped disrupt “dozens” of attacks aimed at the U.S. and elsewhere. According to The Washington Post, Alexander cited the Headley case and promised to make more information public about the success of the NSA’s phone surveillance program, which captures “metadata” such as number, time and location of but not the content of calls.

In January, a federal judge in Chicago imposed a 35-year prison sentence on Headley, 51, for his role in Mumbai and the foiled newspaper plot. He got a reduced sentence because he testified at the federal trial in Chicago of his accomplice, Tahawurr Rana, who was sentenced to 15 years in prison.

Headley confessed to doing undercover surveillance in Mumbai for the Lashkar-e-Taiba terrorist group and Pakistan’s Inter-services Intelligence Directorate (ISI). U.S. officials also charged a major in the ISI with serving as Headley’s handler before the attack in November 2008. Pakistan denies involvement.

In early 2009, according to trial testimony, Lashkar and the ISI sent Headley on a surveillance mission to Denmark. After he returned to Pakistan, his Lashkar and ISI handlers backed off. But Headley continued the plot with support from al-Qaida, whose leaders wanted a team of gunmen to attack the newspaper offices in Copenhagen, take hostages and throw their severed heads out of the windows.

Headley returned to Europe from Chicago for a second reconnaissance mission that July. The official version has been that he was detected at this point — but not by U.S. agencies.

Instead, U.S. and European counterterror officials have told ProPublica in interviews that British intelligence learned of Headley’s contact with al-Qaida operatives near Manchester, England, who were already under surveillance. Headley planned to meet with the extremists in hopes they would supply money, arms and personnel for the Denmark attack.

“Headley was an unknown until not long before his arrest,” a senior U.S. counterterrorism official told ProPublica in 2010. “He came to light because of the British. They knew him only as ‘David the American.’ [The British] MI5 [security service] detected that he was in contact with a group in the U.K. that they were watching … David had made direct contact with two of the main targets of the U.K. investigation.”

On July 23, 2009, the FBI asked U.S. Customs and Border Protection analysts in Washington, D.C., for assistance identifying a suspect who would travel shortly from Chicago via Frankfurt to Manchester, according to U.S. officials interviewed in 2011. The tip described a suspected American associate of Lashkar or al-Qaida with only his first name, flight itinerary and the airline, officials said. The customs analysts identified Headley through their databases containing records of his previous travel and interviews by U.S. border inspectors.

Headley went on to Sweden and Denmark. Alerted by U.S. agencies, Danish intelligence officers followed him as he scouted targets in Copenhagen and tried to find sources for guns, according to court records and interviews with counterterror officials. In the United States, court-approved FBI surveillance continued after his return in August and until his arrest that October, according to counterterror officials and court records.

Officials in Clapper’s office declined to comment on accounts of the British tip. But they said that information lawfully gathered under the Foreign Intelligence Surveillance Act was integral to disrupting the attempted attacks on the Danish newspaper. This does not rule out other sources of information at other points in the investigation, the officials said.

Separately, the U.S. law enforcement official familiar with the case also said this week that a British communications intercept first detected Headley. Because the NSA works closely with its British counterparts, at that point U.S. intelligence agencies likely became involved in reviewing communications records to identify Headley and begin tracking his movements and associates, the official said.

“It was a communications intercept involving a bad guy in England,” the law enforcement official said. “It was the Brits who passed us the info. Without knowing all the gritty technical details, [Clapper's depiction] definitely fits with my understanding.”

The 30,000-page case file in Chicago remains wrapped in secrecy. Prosecutors have not said how investigators first detected Headley. Once he was under investigation by the Chicago field office of the FBI, agents intercepted his calls and emails and retrieved NSA intercepts of previous communications to build the case, according to court documents and ProPublica interviews. During questioning after his arrest, FBI agents confronted him with information from NSA intercepts as well as foreign intelligence agencies, the senior counterterror official said.

“What it may have allowed them to do is to go back and find emails and calls and map his movements,” said Charles Swift, a lawyer for Rana, the Chicago accomplice.

Headley began cooperating after his arrest, turning over his computer and giving the FBI access to his email accounts. Swift said he is not aware of anything in the case to suggest that the disputed NSA programs identified Headley, though he acknowledged that defense lawyers were not shown the government application for a warrant to monitor Headley under FISA.

Swift called the case a dramatic example of the limits of the U.S. counterterror system because both high-tech and human resources failed to prevent the Mumbai attacks.

“You have to know what you are looking for and what you are looking at,” Swift said. “Headley’s the classic example. They missed Mumbai completely.”

The Headley case is also problematic because of his murky past.

The convicted drug smuggler radicalized and joined Lashkar in Pakistan in the late 1990s while spying on Pakistani heroin traffickers as a paid informant for the Drug Enforcement Administration. His associates first warned federal agencies about his Islamic extremism days after the Sept. 11 attacks. Investigators questioned him in front of his DEA handlers in New York, and he was cleared.

U.S. prosecutors then made the unusual decision to end Headley’s probation for a drug conviction three years early. He then hurried to Pakistan and began training in Lashkar terror camps. Although the DEA insists he was deactivated in early 2002, some U.S., European and Indian officials suspect that he remained an informant in some capacity and that the DEA or another agency sent him to Pakistan to spy on terrorists. Those officials believe his status as an operative or former informant may have deflected subsequent FBI inquiries.

The FBI received new tips in 2002 and in 2005 when Headley’s wife in New York had him arrested for domestic violence and told counterterror investigators about his radicalism and training in Pakistan. Inquiries were conducted, but he was not interviewed or placed on a watch list, officials have said.

Headley was recruited in 2006 by ISI officers, who with Lashkar oversaw his missions, according to Headley’s trial testimony and other court records.

In late 2007 and early 2008, another wife told U.S. embassy officials in Islamabad that Headley was a terrorist and a spy, describing his frequent trips to Mumbai and his stay at the Taj Mahal Palace Hotel. In fact, Headley was conducting meticulous surveillance on the Taj and other targets for an impending attack by a seaborne squad of gunmen.

Once again, U.S. agencies say they did not question or monitor him because the information from the wife was not specific enough.

Senior Indian officials believe the U.S. government did not need high-tech resources to spot Headley. They have alleged publicly that he was a U.S. double agent all along. U.S. officials strenuously deny that. They say Headley simply slipped through the cracks of a system in which overwhelmed agencies struggle to track threats and to communicate internally and with each other.

The final tip to authorities about Headley came from a family friend days after the Mumbai attacks. This time, FBI agents in Philadelphia questioned a cousin of Headley’s. The cousin lied, saying Headley was in Pakistan when he was actually at home in Chicago, according to trial testimony and court documents. The cousin alerted Headley about the FBI inquiry, but Headley went to Denmark as planned.

U.S. agencies did not find Headley or warn foreign counterparts about him in the first half of 2009 while he conducted surveillance in Denmark and India and met and communicated with ISI officers and known Lashkar and al-Qaida leaders.