GUANTÁNAMO BAY, Cuba — As the United States restarts its effort to prosecute — and ultimately execute — five detainees accused of conspiring in the Sept. 11 attacks, it has fallen to Brig. Gen. Mark S. Martins both to prove them guilty and to show the world that the tribunal system is now legitimate.

“We’re going to have a fair trial,” General Martins, the chief prosecutor in the military commissions system, said in an interview this week. “There are a lot of people who come to this with preconceptions about unfairness, and I would just ask people to withhold judgment. The initial version of commissions was flawed, but there has been a lot of work on reforms.”

General Martins has assigned himself to lead the latest attempt to prosecute Khalid Shaikh Mohammed — the architect of the terrorist attacks, which killed nearly 3,000 people — and four other detainees who are scheduled on Saturday to be arraigned on war crimes charges at the naval base at Guantánamo Bay, Cuba.

The five had been arraigned at Guantánamo Bay before, in 2008, but the Obama administration shut that case down upon taking office, then tried to move it to federal court in New York, before surrendering to a political uproar.