Pakistani police say an American held after shooting dead two men committed "cold-blooded murder".
The claims came as a judge ordered the man's detention extended for 14 days in a local jail and told the Pakistani government to clarify if he has diplomatic immunity.
The police claims and extended detention are likely to further inflame tensions over the case between the US and Pakistan, whose uneasy partnership is considered key to ending the war in Afghanistan.
The US says the American, 36-year-old Raymond Allen Davis, shot two Pakistanis on January 27 because they were trying to rob him in the eastern city of Lahore.
Washington insists his detention is illegal under international agreements covering diplomats because he was a US Embassy staff member, and American officials have begun curbing diplomatic contacts and threatening to cut off billions in aid to Pakistan if he is not freed.
Pakistani leaders - loathe to incur a backlash in a public already rife with anti-US sentiment - have for days avoided making definitive statements on Davis' legal status, saying the issue is up to the courts. The fact that rival political parties control the federal government and the government of Punjab province, where any trial would be held, is further complicating the Pakistani response.
Judge Anik Anwar ordered that Davis be taken from police custody and held in a local jail for at least two more weeks. In response to defence requests, he also ordered that the government tell the court in the coming days whether the American has diplomatic immunity.
Later in the day, Lahore police chief Aslam Tareen declared that a police investigation into the shootings determined Davis was not defending himself and described it as "an intentional and cold-blooded murder".
The police chief said Davis told interrogators that one of the Pakistani men had pointed his pistol at him, but Mr Tareen said the dead man's pistol had been examined and officers found that all the bullets were in the magazine and no bullet was found in the chamber.
US Embassy spokeswoman Courtney Beale said: "We regret that this incident resulted in loss of life, however eyewitness reports on the day of the incident showed the American diplomat acted in self defence. There is no doubt he has diplomatic immunity, and we are working with the government of Pakistan to resolve the issue."