ISLAMABAD, PAKISTAN - Pakistani police said Friday that investigators have determined that a detained American Embassy official committed "a clear murder" when he fatally shot two Pakistanis last month, a conclusion likely to intensify a diplomatic standoff between the two allied countries.

Raymond Davis, 36, has said he killed the two men in self-defense as they tried to rob him at gunpoint at a busy intersection in the eastern city of Lahore on Jan. 27. U.S. officials have repeatedly insisted he qualifies for diplomatic immunity and must be released.

That prospect appeared to recede Friday morning, however, when a municipal court in Lahore ordered Davis kept in custody for two more weeks. After the hearing, Lahore Police Chief Aslam Tareen told reporters that police did not believe Davis fired in self-defense and that they were preparing murder charges.

"It was a clear murder, and we have proof of that," Tareen said. "He gave no chance of survival to them."

U.S. officials have said that both motorcyclists pointed weapons at Davis and that he thought his life was in danger.

Pakistan, whose weak federal government is under heavy pressure from a strongly anti-American public and religious parties to try Davis, has said the case will be decided in the courts. Some Pakistani government officials, speaking on the condition of anonymity, say Davis will eventually be released to the United States once the furor over the killings subsides - something that has shown little sign of happening.

In retaliation for Davis's continued detention, Washington has suspended some high-level diplomatic contacts with Islamabad and sent signals that the two countries' strategic alliance, which includes billions of dollars in U.S. aid for Pakistan, is under threat.

Following the court's decision Friday to keep Davis in custody, a senior Obama administration official said the White House has decided to call off a planned summit among U.S., Afghan and Pakistani leaders scheduled for Feb. 22 in Washington.

U.S. officials say that Davis, a former Army Special Forces soldier, is a member of the "administrative and technical" staff at the U.S. Embassy in Islamabad, a category that qualifies for diplomatic immunity under the Vienna Convention, and that he was on a temporary assignment in Lahore. They have not explained his duties or why he was armed, though one senior U.S. official described him as a "security official" who was not authorized to carry a weapon.

Tareen said Friday that one of the two men killed by Davis had pointed a pistol at him but that investigators had determined that although the weapon was loaded, there were no bullets in the chamber. The other man was shot in the back as he tried to flee, Tareen said.

Tareen did not clarify how the position of the bullets in the pistol disqualified Davis's claim of self-defense.

According to Pakistani and U.S. officials, the police report of the incident said the two Pakistanis who were killed had robbed two people earlier the same day and stolen their cellphones. The report also said at least one of the motorcyclists cocked a gun and aimed it at Davis while he was stopped at an intersection.

Correspondent Karen DeYoung in Washington and special correspondent Shaiq Hussain in Islamabad contributed to this report.