Kabul, Afghanistan (CNN) — Several Afghan soldiers have been arrested after authorities found a number of suicide vests inside the country’s Defense Ministry, a senior Afghan defense official said Tuesday.

The discovery triggered a lockdown of the ministry, which has since been lifted, said the official, who asked to remain anonymous because he is not authorized to speak to reporters.

The official could not confirm the number of arrests, and there was no comment on the report from the NATO command in Kabul.

Tuesday’s discovery and arrests follow a series of attacks by Afghan security forces on American and allied troops in Afghanistan. So far this year, 16 of the 46 American service members killed in Afghanistan have died in what are called “green on blue” attacks, when Afghan troops turn their weapons on allied forces.