(CNN) -- A U.S.-flagged vessel that pirates stormed off the coast of Nigeria on Wednesday is at a port, a U.S. official told CNN Friday. The official didn't disclose the port's location. A second U.S. official said that 11 people were aboard -- two Americans and nine others -- before it was attacked. Pirates kidnapped the captain and the chief engineer -- believed to be U.S. citizens -- during the attack, a U.S. official previously said. Details about the whereabouts of the 11 have not been released.