Kabul: The Taliban asserted responsibility on Friday for a shooting incident at a military base attached to Kabul’s international airport on Thursday that led to the deaths of three American civilian contractors and wounded a fourth, saying the attacker had infiltrated the ranks of the security forces.

The Pentagon said the three contractors were aircraft mechanics but provided no other details. They worked for Praetorian Standard Inc., or PSI, a small firm based in Fayetteville, North Carolina, with offices in Maryland and Virginia. According to its website, the firm “specialises in providing innovative strategic planning, logistics, operational and security management support services in challenging environments around the world.”

In a statement on Friday, the company confirmed three of its employees had been killed and one other wounded in the attack. The victims, whose names were not released, were working out of the firm’s Kabul office.

“We are shocked by the tragic nature of these deaths and offer our deepest condolences to the families of these brave men,” the statement said.

PSI has worked in Afghanistan since 2010, mostly providing logistics, transportation and security support to a Defense Department and US Geological Survey programme that is exploring potential mineral deposit sources in Afghanistan.

Immediately after Thursday’s attack, suspicion fell on a possible “insider attack” perpetrated by a member of the Afghan security forces who also had access to the military base at the airport. An unidentified Afghan air force official told the Reuters news agency that the shooter was an Afghan soldier.

On Friday, a Taliban spokesman seemed to give credence to that, saying in Twitter messages and a statement that the shooter had joined the Afghan security forces in anticipation of a chance to attack Americans. The attacker was working at Kabul’s airport, the spokesman said.

Taliban spokesman Zabiullah Mujahid referred to the attacker as an “infiltrator,” a term often used by the Taliban for an insurgent who had penetrated the Afghan army or police for months or years, waiting for an opportunity to strike.

“He managed yesterday evening to attain his goal and opened fire with his rifle on a group of American occupiers,” Mujahid said.

When asked Thursday whether the incident was an insider attack, a US military spokesman, Col. Brian Tribus, declined to comment.

Insider attacks have long plagued the relationship between Afghan forces and their US and international allies, breaking down trust and reducing interaction. The assaults by rogue Afghan soldiers or police reached record levels in 2012, when there were 37 such attacks that killed 51 people, including 32 US troops, according to the Pentagon.

Since then, US and coalition forces have tightened vetting procedures for Afghan security forces and required that all foreign troops be armed at all times. The efforts have reduced the number of insider attacks, but they remain a major concern.