CAPE CANAVERAL, Fla. — An unmanned Dragon freighter carrying a stash of precious medical samples from the International Space Station parachuted into the Pacific on Sunday, completing the first official shipment under a billion-dollar contract with NASA.

The California-based SpaceX company successfully guided the Dragon down from orbit to a splashdown a few hundred miles off the Baja California coast. “This historic mission signifies the restoration of America’s ability to deliver and return critical space station cargo,” Elon Musk, the billionaire founder and head of SpaceX, said.

Several hours earlier, astronauts aboard the International Space Station used a giant robot arm to release the commercial cargo ship 255 miles up. SpaceX provided updates of the journey back to Earth via Twitter.

The supply ship brought back nearly 2,000 pounds of science experiments and old station equipment. Perhaps the most eagerly awaited cargo is nearly 500 frozen samples of blood and urine collected by station astronauts over the past year.

The Dragon is the only delivery ship capable of returning items, now that NASA’s shuttles are retired to museums. Atlantis made the last shuttle haul to and from the station in July 2011.

SpaceX — more formally Space Exploration Technologies Corp. — launched the capsule three weeks ago from Cape Canaveral, full of groceries, clothes and other station supplies. Ice cream as well as fresh apples were especially appreciated by the station residents, now back up to a full crew of six.

It’s the second Dragon to return from the orbiting lab; the first mission in May was a flight demo. This flight is the first of 12 deliveries under a $1.6 billion contract with NASA.