Investigators have determined it was a 72mm anti-tank missile that hit the fifth floor of the U.S. AID office, officials said Friday. No one was injured in the attack Thursday evening on the six-story building, which is surrounded by a concrete wall about six feet high. Henry Bassford, head of the local office of the Agency for International Development, said police told him the projectile was fired from about 200 yards. Bassford said the rocket damaged an external wall on the fifth floor of the building in a residential district of the capital. He said U.S. and Salvadoran AID employees were at work at the time but no one was on the fifth floor. No one claimed responsibility for the attack, but officials suspect leftist guerrillas who have been fighting El Salvador's U.S.-backed government for nine years. The Defense Ministry called it an act of terrorism. Military sources said the rocket, known as a LAW anti-tank missile, is a relatively small-caliber weapon readily available on the black market. It was the first attack on an AID office, but the U.S. Embassy in El Salvador has been attacked several times.