The failure of a Russian Mars mission that crashed back to Earth was due to testing errors and engineering flaws, the head of the country's space agency says.

Roscosmos head Vladimir Popovkin said a government commission had completed its investigation of the failure of the $400 million Phobos-Grunt spacecraft.

"The main causes were the errors during production and test works, as well as the engineering flaws," Popovkin said Thursday.

Russia's most ambitious planetary mission in decades, the Phobos-Grunt was launched in November to study the Martian moon Phobos but failed to leave Earth orbit for Mars when its thrusters failed, RIA Novosti reported.

The spacecraft fell back to Earth on Sunday.

There had been speculation in Russia that the failure of Phobos-Grunt could have been caused by a powerful electromagnetic emission from a U.S. radar.

Popovkin said such a possibility was being considered "only as one of the causes."

Roscosmos is expected to hold tests to see whether the probe could have been affected by U.S. radars, Popovkin said.