Somali pirates have freed seven Indian sailors detained for four years in exchange for an undisclosed ransom, Somali officials and a maritime monitoring group said Friday. The sailors, held since pirates hijacked the Panamanian-flagged ship Asphalt Venture in 2010, were freed on Thursday. Eight of their colleagues were freed by the pirates along with the ship in April 2011 for a ransom. Their captors said at the time that they would release the seven sailors only when fellow Somali pirates held by Indian authorities were freed. It was not clear if that demand had been met. Ecoterra International, which monitors maritime activity on the Indian Ocean, confirmed the sailors’ release. At one time, Somali pirates made millions of dollars in ransoms from seizing ships, but increased patrols by international navies on the Indian Ocean have reduced piracy.