For several years, representatives of a major phone company have been sitting beside federal agents in U.S. government offices across the country and passing along certain customer data, ABC News has learned.

The program, used by the FBI, Department of Homeland Security and Drug Enforcement Administration, is primarily intended to let federal agents "more efficiently" respond to lawfully obtained subpoenas and keep up with suspects who routinely swap cell phones, according to a law enforcement official.

Called Hemisphere, it's part of the U.S. government's High Intensity Drug Trafficking Areas - or HIDTA - program, which provides federal money for federal, state and local law enforcement to cooperate in areas deemed "critical drug-trafficking regions," as the White House calls them.

Hemisphere, a "law enforcement sensitive" but not classified effort, puts AT&T contractors in HIDTA offices in Los Angeles, Houston and Atlanta. It responds to requests from any of the other 25 HIDTA offices across the country.

As routinely occurs in investigations, HIDTA agents will seek a grand jury subpoena, administrative subpoena or search warrant directing a phone company to turn over call data about a suspect or person of interest, the law enforcement official said.

But drug traffickers and other criminals often switch phones to evade law enforcement. So under Hemisphere, after a subpoena or warrant is obtained, the AT&T representative will pull data and relay it "in real time" to HIDTA investigators sitting "right there," according to the law enforcement official.

"Hemisphere results can be returned via email within an hour of the subpoenaed request," according to Hemisphere training materials from Los Angeles posted online. "Hemisphere data contains roaming information that can identify the city and state at the time of the call."

Essentially, the program uses a suspect's past phone calls to identify associates, and then uses those associates' recent call patterns to identify the suspect's new number. Subpoenas are obtained to proceed with each step.

Since the Hemisphere program was launched in 2007, the Los Angeles office alone has processed more than 4,400 requests and more than 11,200 phone numbers, according to the materials posted online.

AT&T holds user information dating back to 1987, the law enforcement official said.

The materials posted online say about two-thirds of the requests were related to a "dropped phone," but the rest were "basic" requests to help identify other suspects or conduct "other investigative work," according to the official.

Without providing more detail, the materials say Hemisphere has been used "to track known Canadian phones roaming in the U.S. on the AT&T network." And last year, the program began providing subscriber information and offering "mapping" through certain software.

In May, Hemisphere introduced "limited pinging for some phones," according to the materials, which were intended to help train AT&T representatives participating in the program in Los Angeles, the law enforcement official said.

Asked whether AT&T customers should have more of an opportunity to respond to subpoenas for their information under Hemisphere, the official noted that AT&T can challenge a subpoena under Hemisphere, just as the company can with subpoenas outside Hemisphere. In those cases, the customer is not immediately aware of the subpoena to challenge it either.

The DEA and the Office of Drug Control Policy pay for Hemisphere.

The law enforcement official said the cost amounts to the salaries of the AT&T contractors.

The official also emphasized that Hemisphere is not associated with the U.S. surveillance programs recently disclosed by former National Security Agency contractor Edward Snowden.

"There is nothing classified about issuing a subpoena to a phone company for a drug dealer's phone records," the official said.

Asked for comment, an AT&T spokesman said, "While we cannot comment on any particular matter, we, like all other companies, must respond to valid subpoenas issued by law enforcement."