Islamabad, Pakistan (CNN) -- The American accused of killing two Pakistani men isn't a diplomat, as U.S. officials have said, but rather a CIA contractor in the country providing security for CIA officers, a U.S. government official said Monday.

The revelation that Raymond Davis was working for the U.S. intelligence agency when he shot two men -- one of them in the back, according to Lahore police -- is a dramatic twist in a case that has already inflamed hard-line clerics and an angry public that wants the American tried in a Pakistani court.

In protests since Davis' arrest last month, hard-line Pakistani clerics have condemned the shootings and demanded that the government not release the American.

Despite the revelation of Davis' true line of work, U.S. officials on Monday renewed their argument that he enjoys diplomatic immunity and must be released.

U.S. officials notified Pakistan that Davis had been posted to the U.S. Embassy in Islamabad in January 2010, U.S. officials said. Pakistan's only recourse under international law is to order Davis out of the country, according to a senior U.S. official who briefed reporters on background Monday.

"Any other form of action, including a judicial action or any other proceeding, is inconsistent with his status," the official said.

Davis was jailed January 27 after fatally shooting two men who pulled up to him on a motorcycle in a bustling Lahore neighborhood.

U.S. officials and Lahore police said Davis told them he shot the men in self-defense. Evidence showed the two men may well have been robbers, according to police who found cash and cell phones linked to a robbery reported earlier that day.

But Lahore's police chief, Aslam Tareen, has said witnesses told police that Davis kept firing, even when one of the men was running away.

"It was clear-cut murder," Tareen told reporters.

That Davis was working for the CIA as a contract employee seems to explain the assortment of gear Lahore police reported finding in his rental car following his arrest.

According to a Lahore police report, Davis' car contained 9mm pistol, five ammunition magazines, two cell phones, an infrared light, a digital camera, a telescope, a long-range wireless set and a survival kit.

While acknowledging that Davis is a CIA contractor, the U.S. official said that Davis is not a case officer or paramilitary officer.

"Davis is a protective officer, someone who provides security to U.S. officials in Pakistan. Rumors to the contrary are simply wrong," the official said.

The U.S. official added "any suggestions that he was conducting covert operations or personally leading efforts to target militant groups is wrong."

But the official said Davis could have been providing security for CIA officers who were engaged in covert operations

Davis is part of the agency's "global response staff," which is responsible for protecting the safety of CIA operatives in other countries.

He was doing "advance work," scouting areas of Lahore at the time of the shooting, the official said. Davis' role required him to "know the environment."

He provided all facets of security for case workers including surveillance detection, route surveillance and protection in potentially hostile environments, said the official.

The 36-year-old Davis is a former member of the U.S. Army special forces and had been employed by security firm XE Services, previously known as Blackwater.

Davis began working for the CIA nearly 4 years ago. He was assigned to Pakistan in late 2009. He was living with other security personnel at a safehouse in Lahore before the shooting incident.

Until Monday, U.S. officials had described Davis only as an employee who was attached to the U.S. Embassy in Islamabad and who was working at the U.S. Consulate in Lahore at the time of the shootings.

Formally, they continue to describe him only as a member of the "technical and administrative staff" of the embassy.

The U.S. government has had ongoing conversations with Pakistan's government about Davis' security, State Department spokesman P.J. Crowley said Monday.

"Clearly, we hold the government of Pakistan fully responsible for his safety," Crowley said.

A U.S. official said Davis is being held in a facility with 4,000 other prisoners, some of them militants.

Pakistani officials have moved Davis to a separate part of the prison and taken away guards' guns for fear one might kill the American, the U.S. official said. Dogs are even being used to taste and smell Davis' food to ensure he is not poisoned, according to the official.

Sen. John Kerry, D-Massachusetts, chairman of the Senate Foreign Relations Committee, recently traveled to Pakistan in part to express "our deepest sorrow for the loss of life" and underscore Davis' diplomatic immunity.

Members of Congress have told senior Pakistani leaders that billions of dollars in U.S. aid are in jeopardy unless Davis is released.

But Crowley said Monday no such action is currently being contemplated.

The U.S. supplied more than $1.5 billion in nonmilitary aid to Pakistan in fiscal year 2010.

CNN's Pam Benson and journalist Nasir Habib contributed to this report.