Kabul - Two foreign nationals shot dead in Kabul Saturday were killed by their Afghan guard, who then committed suicide, the Afghan Interior Ministry said Sunday.

The South African and British nationals who worked as director and deputy director for DHL, an international shipping company, were shot dead in front of their office in downtown Kabul.

"Our investigation showed that one of the Afghan guards, who was

on duty yesterday morning, opened fire on DHL's director and deputy director as their vehicle reached the office, killing both on the spot," Interior Ministry spokesman Zemarai Bashary said.

"The investigation also found that the guard then shot himself in the neck," Bashary said.

He said the 13 people, including DHL employees and guards, who were detained by police as part of the investigation, had been released.

"It was not clear if the guard was recruited to carry out the killing, or if some other issues were involved," Bashary said, adding that the police were looking into the background of the attacker.

Police said the guard was working with Saladin Group, a branch of a British-based company that provides security guards to international organizations, banks and private companies.

The company, which has some 2,000 indigenous security personnel, has deployed Afghan security guards throughout the country since 2002.

Officials recently said some security companies - most of which were not registered with the interior ministry - had been behind a spate of crimes including armed robberies and kidnappings.

The Afghan government had started regulating private security firms and has already closed several which were operating illegally, Bashary said. The government had licensed only 60 companies.

Recent shootings have raised concerns for the safety of expatriates working with United Nation organizations, international NGOs and investment companies in kabul.

Saturday's shooting came five days after an aid worker with dual South African and British nationality was killed in Kabul.

Taliban militants took responsibility for the killing of Gayle Williams, 34, who worked for SERVE, a Christian charity working with disabled Afghans.

The militants accused Williams of "preaching Christianity" in the country, an allegation that SERVE denied.

Three Western female aid workers and their Afghan driver were killed in a Taliban attack outside Kabul in August. (dpa)