The pirates, who surrendered without a fight in the North Arabian Sea, were being held aboard the aircraft carrier John C. Stennis, The New York Times reported Friday.
The Iranians were on their way home, the newspaper reported.
The Pentagon said the Iranians had been held for 40 to 45 days in harsh conditions by 15 pirates who used the captured vessel, the Al Molai, as a "mother ship" for subsequent raids.
"They were held hostage, with limited rations, and we believe were forced against their will to assist the pirates with other piracy operations," Josh Schminky, a Navy Criminal Investigative Service agent, said.
The American destroyer USS Kidd, part of a strike group traveling with the Stennis, intercepted the Al Molai Thursday and a team from the destroyer boarded the vessel and freed the crew, the Pentagon said.
"The captain of the Al Molai expressed his sincere gratitude that we came to assist them. He was afraid that without our help, they could have been there for months," Schminky said.