A car bomb discovered and defused outside the U.S. Embassy here Saturday was "a powerful, sophisticated device" that could have destroyed the building's western facade, sources close to the investigation said today.

The bomb, concealed in a stolen car parked 50 feet from the embassy, is the first publicly known instance of a terrorist explosive device in Mexico City in recent times.

Police found and deactivated the bomb 15 minutes before 5 p.m., when the anonymous callers told newspapers that it was scheduled to explode. The street has since been closed to traffic. U.S. officials said they had previously asked Mexican authorities to ban parking there.

A hitherto unknown group calling itself the Simon Bolivar Antiimperialist Commando said the car bomb was planted to retaliate against the "terrorist and genocidal" U.S. air raid against Libya. A communique asserting responsibility for the action, signed by the group and dated April 27, was published today by two Mexico City newspapers.

The U.S. Embassy in Mexico City is located on Paseo de la Reforma, the main thoroughfare. Following a leftist demonstration against the U.S. action in Libya at the embassy gates two weeks ago, U.S. officials suspended the issuing of visas there. The suspension was an expression of U.S. displeasure at what was seen to be inadequate police protection during the protest rally, informed sources said.