(CNN) -- A federal judge on Tuesday threw out a piracy charge against six Somalis in the April attack on the USS Ashland naval ship in the Gulf of Aden.
The ruling by U.S. District Judge Raymond Jackson granted a motion by the defendants to dismiss the piracy charge. They still face other charges in the attack, in which one of their companions was killed.
Jackson noted that the Somali attackers never attempted to board the Ashland. He ruled that the U.S. government failed to demonstrate that any attack on the high seas amounts to piracy.
Other charges still pending against the defendants include assault with a dangerous weapon on federal officers, attack to plunder a vessel, violence against persons on a vessel and use of a firearm during a crime of violence.
Robert Rigney, a lawyer who represented defendant Mohamed Abdi Jama, said Jackson's ruling removed the most serious charge and that he expected the government to appeal.
"Whenever you can get a federal piracy account dismissed by the federal court judge, it is a great thing for my client as well as the others," Rigney said. "But there are still charges pending. We are going to fight the charges."
Dean Boyd, spokesman for the Justice Department's national security division, noted in a statement that the judge denied motions to dismiss the other "very serious" charges in the case.
"We will obviously be moving forward with the prosecution of the case -- and we will consider any options we may have with today's ruling," Boyd's statement said.
The Gulf of Aden is between the Horn of Africa and the Arabian Peninsula. An important maritime route, the gulf has seen a dramatic increase in ship attacks in recent years.
CNN's Adam Levine contributed to this story.