After three successful rocket landings, the leftover SpaceX rocket booster crashed Wednesday while trying to touch down on the company's drone ship.
The attempt came minutes after the Falcon 9 rocket successfully launched two satellites from Cape Canaveral, Florida.
Musk said on Twitter that there was a problem with one of the three engines used to slow the first-stage booster for landing. He said it may be the hardest crash of all. Early attempts also failed.
SpaceX's first successful landing on the platform off Florida's east coast came in April, with two more last month. It also landed a 15-story booster in December on land at Cape Canaveral Air Force Station.