NORFOLK, Va. -- Thirteen Somalis and a man from Yemen pleaded not guilty to piracy, kidnapping and firearms charges Tuesday in the February hijacking of a yacht that left four Americans dead.

They entered their pleas in federal court in Norfolk, where five other men convicted of piracy in a separate case last year had been sentenced to life in prison a day earlier.

"There's nothing I know about these charges. I do not admit that I was a pirate," Mohamud Salad Ali said through an interpreter Tuesday.

Each of the men requested a jury trial and all were ordered to remain jailed until then by Magistrate Judge Tommy Miller, who said they are a threat to society and a flight risk.

A trial has been set for May 17, although prosecutors want it pushed back because trying such cases is complicated.

Pirates have rarely been charged in the U.S. The case against the five Somalis sentenced Monday for attacking the USS Nicholas was the first to go to trial since the Civil War, when a New York jury deadlocked on charges against 13 Southern privateers.

If convicted of piracy against the yacht Quest, the 14 men would face mandatory life sentences without the possibility of parole.

Assistant U.S. attorney Joseph DePadilla said several have already confessed their involvement in piracy to the FBI.

Prosecutors have said other charges are also possible.

The yacht's owners, Jean and Scott Adam of Marina del Rey, Calif., along with friends Bob Riggle and Phyllis Macay of Seattle, were shot to death after they were 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. The pirates are typically motivated by the potential for millions of dollars in ransom money.

The men who pleaded not guilty Tuesday were all in their 20s or early 30s, though at least two said they weren't sure exactly how old they were.

Most said they had little or no education or that it was of little value to them.

"I am illiterate," Mounir Ali, the Yemeni, said through an interpreter who was translating for him through a speaker phone. "I went to school but I did not learn anything."

Several said through an interpreter that they did not understand all of the charges they face, although 22-year-old Ahmed Sala Ali Burale said he did because he is "a wise person."

Each was previously assigned a defense attorney by the court because they couldn't afford them.

The government is prosecuting a separate group of Somali defendants for an alleged April 10 attack on the USS Ashland, also off the African coast. A judge in Norfolk dismissed the piracy charge, but the government is appealing. Oral arguments on the definition of piracy are scheduled for March 25.