Four of the five men accused of plotting the Sept. 11, 2001, attacks have contracted Covid, according to legal staff members, as the coronavirus continued to move through a maximum-security prison for former C.I.A. detainees at Guantánamo Bay.
The prisoner accused of masterminding the attacks, Khalid Shaikh Mohammed, has not tested positive for the virus. But the other four defendants in the capital case have flulike symptoms and are in quarantine, the staff members said.
Another detainee accused of being a terrorist leader, an Indonesian man known as Hambali, who lives on the same tier as Mr. Mohammed, tested positive on Wednesday, forcing the cancellation of meetings between prisoners and defense lawyers scheduled for the rest of the week.
The outbreak apparently began in the adjacent Camp 6 prison, which holds 20 general population prisoners who have been approved for transfer with security assurances.
Then last week, lawyers learned that a prisoner at the maximum-security Camp 5 had flulike symptoms and tested positive for the virus. The virus soon spread to others on his cellblock.
It is believed to be the first coronavirus outbreak at the 70-cell prison housing Mr. Mohammed and 12 others.
The military has declined to comment on the magnitude of the outbreak, among either the 35 wartime captives or the detention operation’s 1,000-member staff of military members and civilians.
Defense lawyers for the former C.I.A. prisoners have expressed concern about their clients throughout the pandemic. Most of those prisoners spent three and four years in agency custody, where they were brutalized at times, and have developed chronic illnesses.
Troops working at the prison are required to be vaccinated. But some of the prisoners are distrustful of U.S. military medicine and have refused vaccines, according to lawyers and legal staff members who declined to be identified because of the classified nature of the prison and the sensitivity of detainee health issues.
The prison has struggled to contain the virus since the start of the pandemic in March 2020. It shut down in-person legal meetings for more than a year and has at times allowed some detainees to consult legal counsel by video feed between the courtroom at Guantánamo and a secret site near the Pentagon.
Clive Stafford Smith, a lawyer for several general population prisoners, said that he traveled to Guantánamo Bay this month for six scheduled meetings and that all but one were canceled because of a Covid outbreak among those prisoners.
Mr. Stafford Smith said he was allowed a single visit with Mohammed Abdul Malik Bajabu, a Kenyan man who has been cleared for transfer with security arrangements.
He left the base after a week of waiting for his meetings to be reinstated, and later heard from Justice Department lawyers that his three other clients had been placed in 10 days of quarantine.