SpaceX came up empty Monday in its search to figure out why an unmanned Falcon 9 rocket exploded minutes after blasting off from a NASA launchpad with a load of space-bound cargo.

The US-made rocket, which was carrying 4,000 pounds (1,800 kilograms) of supplies aboard the Dragon cargo ship, blew up in a cloud of smoke on Sunday, raining down in pieces over the Atlantic Ocean and Florida coast.

"Cause still unknown after several thousand engineering-hours of review," CEO Elon Musk wrote on Twitter early Monday.

He said experts were now focusing on the final milliseconds of the flight in order to determine a cause.

The rocket failure was the third in a series of cargo disasters in the past eight months.

In October, US company Orbital's Antares rocket exploded after launch from Virginia, and in April, Russia lost contact with its Progress cargo ship shortly after liftoff.