Afghanistan Interior Ministry spokesman Zemarai Bashary told CNN it was unclear if the Afghan security guard at a DHL courier office was recruited by Islamic militants to kill the two men, who were the managers of the delivery service's Kabul office.

"We can't say if he was recruited to do the job or there were some other issues," Bashary said, indicating an investigation into the incident was continuing.

He told CNN the guard opened fire on the managers' car as it approached the office, also injuring two other guards before shooting himself in the neck. The DHL office is near the presidential palace.