Three British troops were shot and killed by a man wearing the uniform of an Afghan national police officer in violence-plagued southern Afghanistan, the British Ministry of Defence said Monday.

They were serving with the NATO-led International Security Assistance Force when they were killed Sunday.

The attack is under investigation, the NATO-led force said.

The British said two of the soldiers were from the 1st Battalion Welsh Guards, and the third was serving with the Royal Corps of Signals.

It was unclear whether the attacker was an Afghan police officer or an infiltrator wearing the uniform.

The problem of members of the Afghan forces who have turned their weapons on allied soldiers — so-called “green on blue” attacks — has repeatedly occurred this year.

In February, a top U.S. lawmaker cited at least 42 attacks by Afghan security forces on their international allies in the past five years.

Rep. Buck McKeon, R-California, chairman of the House Armed Services Committee, said 39 of the attacks were by members of the Afghan National Security Forces and three were by contractors.

There was at least one more green-on-blue attack in March when two Americans were shot and killed after reports emerged that Qurans had mistakenly been burned by American troops.

In March, Gen. John Allen, commander of the NATO-led force, noted “an erosion of trust that has emerged from this.” But he said that the systems the Afghans and coalition had put in place to help stop these attacks before they happen were having an effect.

Allen said coalition officials were working on a new procedure to check the backgrounds of Afghans who sign up for the army or police force, and the Afghans “have taken a lot of steps themselves.”

In May, a man wearing an Afghan National Army uniform shot at coalition service members in eastern Afghanistan, killing one of them.

— CNN’s Laura Perez Maestro contributed to this report.