Three Israeli teenagers who were abducted by Palestinians in the occupied West Bank last month were shot at least 10 times with a silenced gun in what appeared to be premeditated killings, a US official involved in the investigation said.
The disclosure clashed with speculation by some Israeli and Palestinian commentators that the captors intended to take hostages for a prisoner exchange but panicked and shot them.
The killing of the three Jewish seminary students followed the collapse of US-brokered peace talks in April.
One of them, 16-year-old Naftali Fraenkel, also held American citizenship.
Israeli police believe the killings led far-right Jews to kidnap and burn to death a Palestinian youth in revenge, and the incident also contributed to an eruption of three weeks of clashes between Hamas fighters in the Gaza Strip and the Israeli military.
Missing since hitch-hiking home on June 12, their bodies were discovered on June 30. Israel blamed Hamas for their deaths but the Palestinian Islamist group has neither confirmed nor denied the allegation.
One of the three Israelis, Gil-Ad Shaer, 16, telephoned police and said "they've kidnapped me!" after he, Fraenkel and 19-year-old Eyal Yifrah got into a car which investigators suspect was driven by a Hamas militant posing as a religious Jew. A second disguised gunman sat in the front passenger seat.
A US official involved in the probe said the FBI, whose mandate includes Americans abducted abroad, received a recording of the distress call from Israel within days and sent it for audio analysis in the United States.
Distorted, tinny reports heard on the tape after an Arabic-accented male voice shouts "head down!" in response to Shaer's attempt to raise the alarm were found to be consistent with shots from a silenced firearm, the US official said.
"There were 10 gunshots," added the official, who was interviewed on condition of anonymity.
The use of a silencer led US investigators to believe the captors planned to kill the three teenagers from the outset, the US official said.
Israeli officials declined to respond to the American account of the investigation, saying it was still ongoing.
"We haven't even caught the kidnappers yet, so we're not going public with anything more now," said an official with the Shin Bet security service, which is in charge of the case.