Eight US Navy personnel aboard the USS Abraham Lincoln carrier strike group attempted suicide during the ships’ deployment against Iran, according to a letter revealed on Wednesday from the service’s acting secretary.

In the letter obtained by CNN, Acting Navy Secretary Hung Cao revealed the number, which had not been previously reported, and it covers crew members from several ships that travel together, including the carrier, in written responses to questions posed by Senator Kirsten Gillibrand about the morale and mental health of the USS Abraham Lincoln’s crew. Cao wrote that no service members had killed themselves during the deployment.

“Since the beginning of Lincoln’s deployment, there have been a total of eight suicide attempts across the strike group,” Cao wrote to Gillibrand, a Democrat and senior member of the Senate Armed Services Committee, in a letter dated Tuesday.

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“This number includes the Lincoln crew and sailors from the carrier air wing, strike group and destroyer squadron staff, escorting destroyers and all embarked aviation squadrons.”

Between 6,000 and 7,000 US Navy sailors were deployed as part of the Lincoln strike group across roughly six ships as the carrier remained out of port for over six months while conducting a high rate of combat operations against Iran.

“Eight attempted suicides is incredibly high,” one US military official told CNN, noting recent carrier strike group deployments have had one or two per year. “The strain of combat ops is much higher than normal deployments. No port calls, constant alerts, etc.”

Following a deployment of 286 days at sea, the USS Abraham Lincoln docked at Thailand’s Laem Chabang Port on September 2.