A U.S.-flagged ship that played a central role in a bloody hijacking drama last spring was attacked again Wednesday, a busy day for piracy in the dangerous waters off the east coast of Africa.

It was the first time a security team aboard a major merchant ship repelled a pirate attack, a top U.S. Navy officer said.

But a defensive weapon that emits a loud noise did not work, Vice Adm. Bill Gortney said in a briefing.

"They tried to employ [a long-range acoustical device] and it did not have the effect," said Gortney, the commander of U.S. Naval Forces Central Command, which is responsible for the area favored by pirates. He said he did not know why the device did not work.

The Maersk Alabama came under automatic weapons fire from pirates about 350 nautical miles east of the Somali coast, the European Union's anti-piracy force said, but fought the attackers off.

The failed attack came on a day of dramatic piracy events: The captain of a ship hijacked Monday was confirmed to have died, even as his ship was used to attack another, and a Spanish prosecutor demanded an investigation into reports that a ransom had been paid for a ship released Tuesday.