Two soldiers were killed and six others were injured on Friday in what military officials called a “tragic accident” at Fort Jackson in South Carolina.

Officials in that Army installation’s public affairs office said on Twitter that two of the six injured soldiers were critically hurt. They said the accident involved “a military vehicle and a troop formation” but did not elaborate.

Fort Jackson officials did not immediately respond to an email on Friday requesting additional information. In their tweet, officials said they would investigate the cause of the accident.