BALTIMORE — One day last year, a trusted courier for Osama bin Laden answered a phone call that might have been wholly unremarkable except for one thing — the National Security Agency was apparently listening in.

That intercepted call helped American intelligence officials track the courier all the way to the walled compound in Pakistan where bin Laden was hiding. The discovery eventually led to last week’s midnight assault by Navy SEALs who killed the al-Qaida leader, ending a pursuit that began in the mid-1990s.

A spokeswoman for the NSA said the agency would not offer more detail, and intelligence officials won’t even confirm the account, which was reported by several news outlets quoting anonymous sources. And yet for the super-secret NSA, one of Maryland’s largest employers with a work force of some 30,000 and a budget in the billions, this singular act of eavesdropping now stands as one of i ts most notable and conspicuous achievements.

While news coverage has largely focused on the raid itself and the Central Intelligence Agency interrogations that yielded the courier’s identity, observers of the U.S. intelligence community say credit also belongs to two intelligence agencies: the NSA in Fort Meade, Md., which scours global communications for clues, and the National Geospatial-Intelligence Agency in Bethesda, Md., which provides mapping and other information.

“That is the important part of the story that has not been fully told yet,” said John V. Parachini, director of the Intelligence Policy Center at the RAND Corp. The two agencies increasingly do much more than gather vast quantities of information, he said: “It’s not just collecting bits and bytes and squeaks and peeps, but putting those into context.”

Rep. C.A. Dutch Ruppersberger of Maryland said bin Laden’s killing has vindicated an intelligence community that endured criticism for years as the terrorist leader eluded the grasp of his American pursuers. The ranking Democrat on the House Permanent Select Committee on Intelligence, Ruppersberger said the bin Laden raid would not have been possible without an array of contributions from various agencies, including the two in Maryland along with the CIA and the military.

“We have now sent a message to the world,” he said. “If you’re going to attack the United States of America, kill Americans, we’re going to find you and bring you to justice.”

Details of the bin Laden raid, and all that led up to it, remain shrouded in secrecy. Marci Green, the NSA spokeswoman, declined to comment. She said NSA’s budget is classified, as is the number of employees who work at the sprawling headquarters off the Baltimore-Washington Parkway.

But published accounts quoting senior government sources, and interviews with analysts and lawmakers who know how the Maryland-based spy agencies operate, create a rough sketch of how the persistence and sophistication of a mammoth government program was able to ferret out information that helped bring down the world’s most wanted man.

The Washington Post published a partial transcript of the phone call, intercepted by the NSA, that put government investigators onto bin Laden’s trail. Citing unnamed U.S. sources briefed on the operation, the newspaper said the call came from an old friend of bin Laden’s courier, who asked him: “What’s going on in your life? And what are you doing now?”

“I’m back with the people I was with before,” he answered, according to The Post’s account.

The friend, realizing that answer probably meant the courier was with bin Laden, replied: “May God facilitate.”

Ruppersberger could not share most of what he knows because it is highly classified. But he was quick to praise the NSA’s eavesdropping capabilities. “Just think of the volume of conversations, both Internet and cellphones, that NSA collects around the world,” he said.

Parachini of RAND said the rule of thumb has been that every six hours, NSA collects an amount of information equivalent to the store of knowledge housed at the Library of Congress.

“The volume of data they’re pulling in is huge,” he said. “One criticism we might make of our (intelligence) community is that we’re collection-obsessed — we pull in everything — and we don’t spend enough time or money to try and understand what do we have and how can we act upon it.”

According to John Pike, director of the private national security group, the NSA relies on powerful computers to flag important conversations, such as the one involving the courier.

“They’re listening for words, phrases, sentences that make no sense — ‘The angry red fox jumps over the moon at dawn.’ “ In addition to coded conversations, the computers also listen for obvious red flags like “bomb,” “plot” and “jihad.”

Pike says it would have been exceedingly careless for the courier, identified as Sheikh Abu Ahmed, to use a phone at all, considering al-Qaida’s well-known avoidance of telephones. The NSA was reportedly monitoring not Ahmed but whoever it was who called him, and stumbling across bin Laden’s courier was a stroke of good luck for the government.

Once American eyes were trained on the Abbottabad compound, the lack of a phone line or Internet access on the property would have challenged NSA’s eavesdroppers. But Pike theorized about other spying methods that may have been used.

According to one story Pike heard from his intelligence sources, American officials concluded there was one person at the compound who never ventured outside — presumably bin Laden. Pike described how he thinks agents might have reached that conclusion, saying it sounds like the work of the Special Collection Service, a joint NSA-CIA surveillance operation.

“I would do that by having a Special Collection Service team get an apartment a mile away and start shining laser beams on all the windows,” he said. “The voice noise on the inside of the room is going to cause window glass to vibrate. A laser beam illuminates the window glass and detects the vibration.”

By analyzing the vibrations, he said, analysts could tease out the number of distinct voices. “I can do voice identification to count how many people I’m listening to. If I count 22 people inside the building and 21 outside the building, I know I’ve got somebody who never goes outdoors.”

The New York Times reported that for months the CIA had used a nearby rented house to watch and photograph people at the compound.

The newspaper said CIA agents used cameras with telephoto lenses and infrared imaging equipment. It also said they used sensitive eavesdropping equipment in an attempt to detect voices inside the building and to track any cellphone calls.

(Possibly contradicting the story Pike heard, the surveillance team reportedly saw a man take regular walks in the compound’s courtyard, though they could not confirm that was bin Laden.)

The Times also reported that a satellite used radar to search for possible escape tunnels. Such imagery is supplied by the National Geospatial-Intelligence Agency, known as the NGA.

Ruppersberger credited the NGA with providing a wealth of images and other clues that helped military planners prepare for the helicopter assault. As he put it, “You had to know exactly where you were, what you were doing, how many stairs you had to climb, how many people were in there.

“To think we could go in there in 45 minutes, including blowing up one of our own helicopters — it’s amazing,” he said. “Pakistan, by the time they realized something was happening and were getting their jets up, we were out of there.”