Akhtar Mohammad, an employee of the company, said among those killed was the head of the company's Kandahar office, who he identified as Mr Jack, AIP reported.
US Protection and Investigations (USPI), based in Texas, is one of the biggest security firms working in Afghanistan. It has operated there since 2002. It employs 45 expatriates and hundreds of Afghans.
The blast was the sixth suicide attack in Kandahar province in the past nine days.
Qari Mohammad Yousuf, a spokesman for the Taliban, told Reuters news agency that the bomber was a member of the group.
The area where the attack happened is opposite the Canadian Provincial Reconstruction Team inside Kandahar city.
About 105 suicide bomb attacks this year have killed nearly 250 people, most of them civilians, according to the Nato force in Afghanistan.