A Somali man who secretly pleaded guilty to terrorism charges in New York two years ago continues to provide information to the US government in what officials describe as an "intelligence watershed", the Guardian has learned.

Ahmed Abdulkadir Warsame has yet to be sentenced and transferred to federal prison for his inevitable incarceration, something lawyers say is rare so long after pleading guilty.

But Warsame's extended co-operation, confirmed to the Guardian by his lawyers, helps underscore why US officials consider his bizarre saga a potential template for counter-terrorism that relies less on targeted killings and more on captures, as Barack Obama pledged in May. At least one recent capture operation in Libya appears modeled on Warsame.

"Mr Warsame is co-operating with the US government and as is standard in all co-operation agreements, he will not be sentenced until the government has decided it no longer requires his assistance," Priya Chaudhry, one of Warsame's lawyers, said.

The Justice Department would not answer questions about the nature of the information Warsame continues to provide. But legal analysts and former counter-terrorism officials say it can span the gamut from intelligence about the mutations in the al-Qaida affiliate network to testimony against specific terrorism defendants.

"Someone with his level of access would have information on the details and nuances of the overall structure and that of some of the al-Shabaab branches and offshoots," said Robert McFadden, a former terrorism investigator with the Naval Criminal Investigative Service.

"In a carefully calibrated manner, he might help in explaining – and perhaps validating – recent past and current intelligence the US and allies are looking at."

Warsame is unlike any of the hundreds of suspected or convicted terrorists captured by either the US military or law enforcement. In April 2011, US special operations forces intercepted him at sea as he attempted to return to Somalia from Yemen. With the Obama administration unwilling to add to the detainee population at Guantánamo Bay, Warsame spent over two months in the brig of the USS Boxer, the first known case of a naval ship being repurposed for prolonged detention and interrogation.

Warsame was interrogated aboard the Boxer, performed by an FBI-led team of specialists, before being Mirandized and questioned by a so-called "clean team", which extracted information to be used against him in his legal proceedings.

Extreme secrecy has characterized the Warsame case from the start. His stay aboard the Boxer was a secret until Warsame arrived in a New York jail and the Justice Department announced his indictment on nine terrorism-related counts in July 2011. Nearly all his court transcripts and trial records are sealed to the public. Warsame's lawyers have declined to give additional information about their client, citing court-mandated gag orders.

Nearly as soon as Warsame's case became public, US officials described the information he provided as majorly significant, covering the activities of al-Qaida and al-Shabaab in Yemen and the Horn of Africa, as well as connections between the two organizations and their operatives.

Officials have said Warsame met with Anwar al-Awlaki, an American citizen who became a top al-Qaida propagandist. Awlaki was killed six months after Warsame's capture after a years-long hunt, although it is unknown whether or to what degree Warsame played a role in the September 2011 strike that killed Awlaki.

The public heard little about Warsame between his indictment and March 2013, when Preet Bhahara, the US attorney for the southern district of New York, announced that Warsame had secretly pled guilty in December 2011 on charges including the provision of material support to al-Shabaab and al-Qaida in the Arabian Peninsula; using explosives and firearms; and "receiving military-type training from a foreign terrorist organization".

The maximum and minimum sentences carried by the charges to which Warsame pled guilty were the same: life imprisonment.

Under the terms of the plea deal, Warsame was to "truthfully disclose all information with respect to the activities of himself and others concerning all matters about which this office inquires of him, including all foreign intelligence known to him, which information can be used for any purpose."

The US attorney's office for the southern district of New York declined to comment beyond referring to Bharara's statement announcing Warsame's plea, which said Warsame yielded an "intelligence watershed".

Dean Boyd, a former Justice Department spokesman now with the CIA, told CNN in March that the US was making "active use" of Warsame's intelligence, and that his "co-operation has been and continues to be enormously valuable."

However accidental or rare Warsame's circumstances are, the case is seen within the Obama administration as a wild success, proving the disputed political point that criminal prosecutions do not jeopardize the active collection of sensitive intelligence.

In October, US forces captured Abu Anas al-Liby off the streets of Tripoli and sent him via navy ship to New York to stand trial for his alleged role in the 1998 US embassy bombings in Kenya and Tanzania that killed more than 220 people. Though Liby's case differed from Warsame's in some respects – the US announced that it had happened the weekend it occurred instead of keeping it hidden until Liby reached New York – legal experts noted its similarity to the Somali informant's.

"The United States appears to be following the hybrid approach to counter-terrorism developed in the case of Ahmed Abdulkadir Warsame," former air force colonel and Guantánamo defense lawyer David Frakt wrote shortly after Liby's capture.

Frakt called the Warsame model "an excellent example of how military, intelligence and law enforcement assets can be utilized to efficiently remove a terrorist threat from circulation in a legally supportable way while still eliciting valuable intelligence, and is a good template for the United States to follow in Liby's case."

Rare as it is for an admitted terrorist to have his sentencing deferred as he co-operates with the government, it is not unprecedented. L'Houssaine Kerchtou, one of al-Qaida's earliest members, pleaded guilty to a terrorism charge in 2000, testified at another terrorism trial as late as 2010, and has yet to be sentenced.

Karen Greenberg, the director of Fordham University Law School's Center on National Security, said the Warsame case demonstrates how prosecutors are expanding the boundaries of the law for counter-terrorism – partially to show that civilian legal structures are an effective alternative to military detention and interrogation.

"The law is flexible enough and elastic enough to allow this," Greenberg said.

"You're beginning to see the way the system is adapting to get the kind of intelligence in a detention situation – albeit in a shorter period of time – that was the original premise for Guantánamo."