The U.S. military’s cyberwarfare unit is scrutinizing an unusually high number of deaths by suicide among personnel over a monthlong period this summer, according to government officials and other people familiar with the matter.

The deaths have worried U.S. lawmakers and military leaders within the secretive section of the armed forces, which defends American computer systems and hacks foreign adversaries, according to the people and records reviewed by reporters. They’ve also resurfaced officials’ longstanding concerns about the mental health of military hackers who have over the last year seen their workload surge because of foreign conflicts.

Between early June and early July, as many as five people who worked in or closely with U.S. Cyber Command took their own lives, according to internal military communications, public records and the people. They spoke on condition that they not be identified because they weren’t authorized to discuss the matter.