Twelve years later, the cranes and earthmovers around the National Security Agency are still at work, tearing up pavement and uprooting trees to make room for a larger workforce and more powerful computers. Already bigger than the Pentagon in square footage, the NSA’s footprint will grow by an additional 50 percent when construction is complete in a decade.

And that’s just at its headquarters at Fort Meade, Md.

The nation’s technical spying agency has enlarged all its major domestic sites — in Colorado, Georgia, Hawaii, Texas and Utah — as well as those in Australia and Britain.

Since the attacks of Sept. 11, 2001, its civilian and military workforce has grown by one-third, to about 33,000, according to the NSA. Its budget has roughly doubled, and the number of private companies it depends on has more than tripled, from 150 to close to 500, according to a 2010 Washington Post count.

The hiring, construction and contracting boom is symbolic of the hidden fact that in the years after the Sept. 11 attacks, the NSA became the single most important intelligence agency in finding al-Qaeda and other enemies overseas, according to current and former counterterrorism officials and experts. “We Track ’Em, You Whack ’Em” became a motto for one NSA unit, a former senior agency official said.

The story of the NSA’s growth, obscured by the agency’s extreme secrecy, is directly tied to the insatiable demand for its work product by the rest of the U.S. intelligence community, military units and the FBI.

The NSA’s broad reach in servicing that demand is at the heart of the controversy swirling around the agency these days. Both Congress and the public have been roiled by the disclosure of top-secret documents detailing the collection of U.S. phone records and the monitoring of e-mails, ­social-media posts and other Web traffic of foreign terrorism suspects and their enablers.

Lacking a strong informant network to provide details about al-Qaeda, U.S. intelligence and the military turned to the NSA’s technology to fill the void. The demand for information also favored the agency’s many surveillance techniques, which try to divine the intent of people by vacuuming up and analyzing their communications.

“There was nothing that gave you more insight into the inner workings of these organizations as the NSA,” said Michael Leiter, former director of the National Counter­terrorism Center. “I can’t think of any terrorist investigation where the NSA was not a pre­eminent or central player.”

One top-secret document recently disclosed by former intelligence contractor Edward Snow­den, who is on the run from U.S. authorities, revealed that 60 percent of the president’s daily intelligence briefing came from the NSA in 2000, even before the surge in the agency’s capabilities began.

“The foreign signals that NSA collects are invaluable to national security,” the agency said in a statement released Friday to The Post. “This information helps the agency determine where adversaries are located, what they’re planning, when they’re planning to carry it out, with whom they’re working, and the kinds of weapons they’re using.”

The NSA’s ability to capture, store and analyze an ever greater amount of people’s communications has never been accompanied by public explanations of new legal authorities, programs or privacy safeguards. Only the unauthorized disclosure of these secrets has forced officials to explain them in broad terms, reassure the public and complain about the damage from their public airing.

“I wish that I were here in happier times for the intelligence community,” said Robert S. Litt, general counsel for the Office of the Director of National Intelligence, speaking at the Brookings Institution on Friday. “These disclosures threaten to cause long-lasting and irreversible harm to our ability to identify and respond to the many threats facing our nation.”

Battlefield support

The story of the NSA’s post-Sept. 11 history could begin in many places, including the parking lot of the CIA. There, in late 2001, a burly Navy SEAL paced inside a trailer with a telephone to his ear. The trailer had been hastily converted from a day-care facility to an operations center for the CIA’s covert armed drone program, which was about to kill one of its first al-Qaeda targets, 8,000 miles away in Afghanistan.

On the line with the SEAL was the drone operator and a “collector,” an NSA employee at the agency’s gigantic base at Fort Gordon in Augusta, Ga. The collector was controlling electronic surveillance equipment in the airspace over the part of Afghanistan where the CIA had zeroed in on one particular person. The SEAL pleaded with the collector to locate the cellphone in Afghanistan that matched the phone number that the SEAL had just given him, according to someone with knowledge of the incident who spoke on the condition of anonymity because of the sensitivity of the issue.

The collector had never before done such a thing. Before even intercepting a cellphone conversation, he was accustomed to first confirming that the user was the person he had been directed to spy on. The conversation would then be translated, analyzed, distilled and, weeks later, if deemed to be interesting, sent around the U.S. intelligence community and the White House.

On that day, though, the minutes mattered.

“We just want you to find the phone!” the SEAL urged. No one cared about the conversation it might be transmitting.

The CIA wanted the phone as a targeting beacon to kill its owner.

The NSA collector in Georgia took what was then considered a gigantic leap — from using the nation’s most sophisticated spy technology to record the words of presidents, kings and dictators to using it to kill a single man in a terrorist group.

The revolutionary significance of that and other similar operations was quickly grasped by intelligence officials. With analysts and technicians from the National Geospatial-Intelligence Agency, the NSA subsequently assembled a team in the basement of its headquarters called the Geolocation Cell, or Geo Cell. Its purpose was to track people, geographically, in real time.

The cell opened up chat rooms with military and CIA officers in Afghanistan — and, eventually, Iraq — who were directing operations there. Together they aimed the NSA’s many sensors toward individual targets while tactical units aimed their weaponry against them.

A motto quickly caught on at Geo Cell: “We Track ’Em, You Whack ’Em.”

With the 2003 invasion of Iraq, and the surprisingly quick disintegration of postwar conditions there, the NSA began sending collectors with surveillance equipment to embed with Army brigades and Marine regimental combat teams to target insurgents and terrorists. The units were called tactical cryptologic support teams. The military commanders often had no prior understanding of what the NSA did. But they quickly demanded more of the agency once they learned what it could do.

At the same time, the NSA supported a parallel effort by CIA paramilitary units and clandestine Joint Special Operations Command (JSOC) teams tasked with capturing or killing al-Qaeda leaders, deemed “high-value targets.” NSA analysts and collectors moved into the JSOC commander’s new and growing operational headquarters in Balad, Iraq, which also serviced Afghanistan.

By September 2004, a new NSA technique enabled the agency to find cellphones even when they were turned off. JSOC troops called this “The Find,” and it gave them thousands of new targets, including members of a burgeoning al-Qaeda-sponsored insurgency in Iraq, according to members of the unit.

At the same time, the NSA developed a new computer linkup called the Real Time Regional Gateway into which the military and intelligence officers could feed every bit of data or seized documents and get back a phone number or list of potential targets. It also allowed commanders to see, on a screen, every type of surveillance available in a given territory.

Air Force Gen. Michael V. Hayden, former director of the NSA, said in an interview last week that he would tell people, “If we could do this half well, this will be the golden age of sigint,” or signals intelligence.

A growing reach

The battlefield technology overseas was matched by a demand back in the United States for larger amounts of data to mine using the NSA’s increasingly sophisticated computers. Financial and biometric data, the movement of money overseas, and pattern and link analysis became standard NSA tools. Another example, recently revealed by Snowden, is the bulk collection of telephone metadata — information about numbers dialed and the duration of the calls.

The NSA’s burgeoning secret activities splashed into public view in 2005 when the New York Times reported on the warrantless surveillance of U.S. communications, and subsequent statements by former NSA employees contended that the agency was collecting Americans’ e-mails and phone calls. Some suspected that NSA capabilities were limitless when it came to counterterrorism investigations.

Although the NSA tries hard to maintain a low profile, the physical manifestation of its growing importance has been quietly evident to the communities that surround its major foreign and domestic bases.

Within the past couple of years, bulldozers have plowed through the earth near Bluffdale, Utah, to ready a million-square-foot facility housing a center that will store oceans of bulk data.

In 2007, ground was broken for a $1 billion facility on 120 acres at Fort Gordon, where an NSA workforce of 4,000 collects and processes signals intelligence from the Middle East, according to the agency.

In Hawaii, the NSA outgrew its Schofield Barracks Army site years ago and opened a 250,000-square-foot, $358 million work space adjacent to it last year. The Wahiawa Annex is the last place that Snowden, then a contractor for Booz Allen Hamilton, worked before leaving with thousands of top-secret documents. The main job of the NSA’s Hawaii facility is to process signals intelligence from around the Pacific Rim.

In Texas, the agency has added facilities to its San Antonio-based operations. Its main site, at Lackland Air Force Base, processes signals intelligence from Central and South America. In Colorado, the NSA’s expanding facilities on Buckley Air Force Base in Aurora collect and process information about weapons systems around the globe.

Overseas, the NSA’s station at RAF Menwith Hill on the moors of Yorkshire is planned to grow by one-third, to an estimated 2,500 employees, according to studies undertaken by local activists. Although hidden from the main road, up close it is hard to miss the 33 bright-white radar domes that sprout on the deep green landscape. They are thought to collect signals intelligence from parts of Europe, the Middle East and North Africa.

The NSA’s Pine Gap site in Australia has added hundreds of new employees and several new facilities in recent years. Over the years, Pine Gap has played a role in many U.S. and NATO military operations, including intercepting communications about possible nuclear testing by the Soviet Union during the Cold War and an analysis of the technical characteristics of Iraq’s GPS jamming systems during the 2003 invasion, according to a book by David Rosenberg, a former NSA analyst at Pine Gap. It also processes signals intelligence from parts of Asia.

The upgrades to the cryptologic centers were done “to make the agency’s global enterprise even more seamless as we confronted increasingly networked adversaries,” according to the NSA statement to The Post. “However, we always adjust our efforts to exploit the foreign communications of adversaries and defend vital U.S. networks in accordance with national priorities and in full accordance with U.S. law.”

It added: “The notion of constant, unchecked, or senseless growth is a myth.”

Julie Tate contributed to this report.