Five American service members were killed in southern Afghanistan in a rare friendly fire airstrike that hit a team of Afghan and U.S. troops conducting a security operation ahead of Saturday’s presidential runoff election, U.S. and Afghan officials said Tuesday.

It marked the deadliest day for U.S. troops in Afghanistan since a helicopter crash in December in the same province, Zabul, killed six service members. U.S. military officials said they are investigating Monday’s incident.

The team of Afghan and U.S. troops had been patrolling the Arghandab district and was preparing to leave the area by helicopter when it came under attack from militants firing rockets and small arms, provincial police chief Ghulam Sakhi Roghlewanai said.

Seeking to beat back the assault, the U.S. troops called in an airstrike. Ammunition dropped from a B-1B bomber appears to have killed the Americans, according to an official briefed on the preliminary investigation who was not allowed to speak on the record. At least two of the casualties were Special Operations troops, the official said.

Officials said it was too early to conclude whether the incident was the result of aircraft malfunction, pilot error or miscommunication between troops on the ground and those in the aircraft.

“Investigators are looking into the likelihood that friendly fire was the cause,” the Pentagon’s press secretary, Rear Adm. John Kirby, said in a statement. “Our thoughts and prayers are with the families of these fallen.”

At a news conference Tuesday, Kirby confirmed the five U.S. deaths but did not provide details about those killed. He said there were no reports of any U.S. troops wounded in action.

“We need to let investigators investigate,” he said, declining to comment further.

Roghlewanai said one Afghan soldier was killed in the operation.

“There were no Taliban casualties from the airstrike, but we had killed many of them during the day’s operation,” he said.

A relatively small number of friendly fire incidents in the war have occurred during firefights. Some involved airstrikes in which pilots mistook friendly forces for militants. Monday’s incident appeared to be among the deadliest involving U.S. troops, according to a review of past cases.

With the U.S. withdrawal from Afghanistan ramping up as the end of the American combat mission nears, U.S. troops are doing far less fighting and conducting fewer patrols than in years past. But service members deployed in the south and the east continue to come under attack.

U.S. officials see Saturday’s runoff as a turning point in the war and have expressed concern that violence could mar the legitimacy of the vote. The Taliban has called the election a Western-designed charade and has vowed to derail it with violence.

The group carried out high-profile attacks in the lead-up to the first round of balloting, held in April. Last week, the leading candidate narrowly escaped an assassination attempt, underscoring the country’s fragility as it prepares for its first democratic transfer of power and the pullout of foreign combat troops by the end of the year.

The candidate, former foreign minister Abdullah Abdullah, was unharmed and vowed to press ahead with his campaign. He faces former finance minister Ashraf Ghani in the runoff.

There have been fewer spectacular attacks in recent weeks, but clashes between militants and Afghan security forces remain an everyday occurrence.

On Tuesday morning, the Taliban kidnapped 33 university professors on Afghanistan’s most important highway. The professors were on a bus headed north on Highway 1, which connects Kandahar to Kabul, when they got caught in a firefight between insurgents and Afghan security forces­ in eastern Ghazni province, according to Afghan officials and the professors’ colleagues at Kandahar University. At some point, the Taliban fighters abducted the busload of civilians.

University officials are working through tribal elders to secure the release of the professors, said Hazrat Totakhel, the chancellor of Kandahar University. Most of the professors were on their way to visit family members, he said. It is unclear where and why they are being held.

The kidnapping is jarring not only for its scale, but also for its location — on a thoroughfare that U.S. and Afghan troops have fought for years to defend. In addition to the 33 kidnapped, at least one professor was wounded in the crossfire, Totakhel said.

Ahmadullah Ahmadi, the deputy governor of Ghazni, said he expected the men to be released as early as Wednesday after early contacts with the Taliban.

Sharif Mohammad and Kevin Sieff in Kabul and Dan Lamothe in Washington contributed to this report.