The U.S. military believes that an air strike against the Islamic State in Libya last week did not kill two kidnapped Serbian diplomats, as Belgrade has asserted, and instead suspects the Westerners were kidnapped by a separate criminal group that killed the foreigners and tried to pass off their deaths.

Capt. Jeff Davis, a Pentagon spokesman, said the Defense. Dept. was reviewing reports surrounding the Feb. 19 strike on a suspected Islamic State camp outside the Libyan city of Sabratha, which killed at least 40 people.

“Thus far, we have not found any credible information that indicates these people were killed in this air strike,” Davis said, referring to Serbian diplomats Sladjana Stankovic and Jovica Stepic, who were kidnapped in Sabratha on Nov. 8th.

The powerful strike, which targeted a senior militant suspected of plotting external attacks, was a reflection of mounting Western anxiety about the Islamic State’s Libya branch, seen as the most formidable outside of Iraq and Syria. In recent weeks, U.S. and European officials have accelerated discussions aboutmilitary action against the group, along with potential stabilization activities if Libya’s two rival governments can strike a deal to end a long political feud.

Hours after the U.S. raid, a local security force loyal to the government in Tripoli said it had transported the Serbian diplomats’ bodies to an airport there for investigation. In a post on its Facebook page, the Special Deterrence Force, which is loyal to the Tripoli government, showed two green coffins in the back of a van, along with photos of the embassy workers.

The next day, Serbian Prime Minister Aleksandar Vucic said that the diplomats had been killed in the American bombing, becoming what he called “terrible collateral damage.” Serbian Foreign Minister Ivica Dacic said his government believed the two hostages might have been soon freed.

“I believe we had been close to the solution for them to be freed. Unfortunately, as a consequence of the attack against Isis in Libya, the two of them lost their lives,” Dacic said, according to the Associated Press.

A defense official, who like others spoke on condition of anonymity to discuss an internal assessment, said the Pentagon had several reasons for believing the Serbians were not at the Islamic State camp when the strike place. First, he said, Libyan first responders who arrived at the bombing site and later took casualties to a nearby hospital had not reported finding Westerners on the scene.

Secondly, he said, photos circulated of the slain diplomats did not appear to have been taken at or near the bombing site, and did not suggest either was killed in what the official described as a massive strike.

“The state of the remains was not consistent with having been killed in an airstrike of this magnitude and intensity,” the official said.

Instead, American officials said they believe the diplomats may have already been dead, and their bodies may have already been in Tripoli, at the time of the airstrike. Officials said they believed the Westerners were not being held hostage by the Islamic State but rather by a criminal group that hoped to ransom them for cash.

Five years after Libyans first rose up against former dictator Moammar Gaddafi, the country is a dangerous place, where multiple militia factions, Islamist groups and criminal networks operate.

“There was never any indication they were taken by the Islamic State,” the defense official said. The Serbian government acknowledged that the diplomats’ kidnappers had requested ransom payment, but said it had not paid one.

The Pentagon has previously suggested that it had no information to confirm the Serbian government’s assertion but had not provided an explanation for what occurred.

Ivana Mangov, an official at the Serbian embassy in Washington, declined to say whether the government in Belgrade agreed with the American assessment. She said the Serbian Military Medical Academy had conducted an autopsy on the slain diplomats’ bodies, which were brought back to Belgrade on Tuesday.

“We would like to underline that the circumstances of the tragic death of Serbian diplomats in Libya is a highly sensitive issue,” she said. “We have to respect the fact that the investigation on this case is still ongoing.”

A spokesman for the government in Tripoli did not immediately respond to a request for comment.

While the U.S. military seeks to avoid unintended casualties in remote strikes, its precautions are not fail-safe. Last year, the White House acknowledged the United States had killed Warren Weinstein, a kidnapped American aid worker, when a CIA drone strike in Pakistan. In 2014, the U.S. government paid over $1 million in compensation for a drone strike that hit a wedding party in Yemen.