A Yemeni man pleaded guilty to piracy on Thursday for his role in the hijacking of a yacht off the coast of Africa that left four Americans dead.

Mounir Ali is now the 11th man to plead guilty in the February hijacking of the yacht Quest, although prosecutors don't believe any of those men fired the fatal shots aboard the sailing vessel.

The owners of the Quest, Jean and Scott Adam of Marina del Rey, Calif., along with friends Bob Riggle and Phyllis Macay of Seattle, were shot to death after being taken hostage several hundred miles south of Oman.

They were the first U.S. citizens killed in a wave of pirate attacks that have plagued the Gulf of Aden and the Indian Ocean in recent years, despite an international flotilla of warships that patrol the area. Four U.S. warships were shadowing the Quest and negotiations were underway when shots aboard the sailing vessel were fired.

Court documents have identified three Somalis at the triggermen aboard the boat. They each still face piracy, kidnapping and weapons charges, and prosecutors have said additional charges are likely in the future.

Ali faces life in prison at sentencing in October, although that could later be reduced as part of a plea agreement that requires him to help prosecutors in this case and possibly others.

Ali had previously rejected the plea deal because he said he was forced to join the Somalis after the boat he was on was hijacked. His attorney, Jim Theuer, said he advised Ali he likely wouldn't prevail at trial.

Theuer said although Ali may have been under duress when he was initially captured, it would be difficult to prove he was still under duress when he boarded the American boat. Each of the 10 Somali men who pleaded guilty said in court documents that Ali willingly decided to join them for a share of the ransom profits.

Theuer said he believes that anyone who didn't take the plea deal, which includes eventually having weapons and kidnapping charges dismissed, could face capital charges in the future.

Only four other men are charged in the case — the three identified as the shooters — and a land-based negotiator who the U.S. says is the highest-ranking pirate it has ever captured.

The negotiator, Mohammad Saaili Shibin, is unlike the other men charged in connection with the case because he never set foot aboard the Quest. Court documents say he researched the Americans online to determine how much of a ransom to seek for them.

On June 21, a federal judge denied a request from Shibin to have $1,600 in cash returned to him for humanitarian needs, saying he failed to prove he was lawfully in possession of the money.

The money in question was seized from Shibin when he was arrested in Somalia in April.

Prosecutors believe the cash may be tied to a ransom payment he received for negotiating the release of a German ship, the M/V Marida Margerite. Court documents say Shibin was unemployed at the time he made $37,000 in deposits soon after the German vessel was released in December.

"Curiously, defendant does not indicate how he came upon $1,620 in United States currency, an explanation which might have proven useful to the court given that he was allegedly unemployed in 2010," U.S. District Judge Robert Doumar wrote in his order.

No charges have been filed in the German case, although prosecutors wrote they anticipate filing a mutual legal assistance request with Germany for evidence in it. Prosecutors said evidence from the Germans may form the basis for additional charges against Shibin.