The director of the National Security Agency Gen. Keith Alexander told a Senate hearing on Wednesday that phone records obtained through the secret surveillance programme disclosed last week helped to prevent "dozens" of terrorist acts.
Among the specific cases he cited was that of Pakistani-American Headley, who was arrested in 2009 for his role in the Mumbai attack, and who was plotting to attack a Danish newspaper that published a satirical cartoon of the Prophet Mohammed.
Earlier James Clapper, the director of national intelligence, had said the programme helped stop an attack on Denmark's Jyllands-Posten newspaper for which Headley did surveillance.
The Senate intelligence chairperson Dianne Feinstein also called Headley's capture a success.
But a closer examination of the case, respected investigative journalism site ProPublica said, shows that the government surveillance only caught up with Headley after the US had been tipped by British intelligence.
"And even that victory came after seven years in which US intelligence failed to stop Headley as he roamed the globe on missions for Islamic terror networks and Pakistan's spy agency," it said.
"Headley's case shows an alarming litany of breakdowns in the US counter-terror system that allowed him to play a central role in the massacre of 166 people in Mumbai, among them six Americans," ProPublica said.
Headley avoided arrest despite a half dozen warnings to federal agents about extremist activities from his family and associates in different locales, the news site said.
The news site cited unnamed senior Indian officials as saying they believe the US government did not need high-tech resources to spot Headley.
US officials have strenuously denied Indian allegations that Headley was a US double agent.
"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," ProPublica said.
The final tip to authorities about Headley came from a family friend days after the Mumbai attacks, it noted.
US 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 officers of Pakistan's Inter-Services Intelligence (ISI) and known Lashkar-e-Taiba and Al Qaeda leaders.