ROME (AP) -- Gunmen waiting in ambush shot and killed a top government labor official as he left for work Thursday, in what Italian political leaders were quick to call an apparent act of terrorism. The killers -- at least two young men -- lay in wait for Massimo D'Antona in a stolen van outside his home near the city center. They opened fire when he stepped outside, hitting him at least three times in the chest, authorities said. Interior Minister Rosa Russo Jervilino said police found two .38 caliber guns at the scene of the crime. ``The circumstances of the killing have not yet been reconstructed in detail, but from what we know so far we can say that there is little doubt of the precise preparation of the ambush and of the cold and professional execution,'' she said. There was no immediate word on a possible motive. Suggestions ranged from a protest against Italy's participation in NATO to D'Antona's standing as a reformer. Investigators were investigating a claim of responsibility to Rome's Il Messagero daily in the name of the feared leftist Red Brigades. Other claims came on behalf of the lesser-known Armed Communist Cells and the little-known right-wing Armed Phalanx. Jervilino called an emergency meeting of the national security commission and Walter Veltroni, the leader of the center-left governing coalition's main party, urged a state of maximum alert. ``I have the plain sense of a return to the years of the darkest period of our country,'' said Veltroni, head of the Democrats of the Left. Veltroni was one of many to liken the killing to those of the ``Years of Lead'' -- the 1970s and 1980s, when Red Brigade bullets killed hundreds in Italy. D'Antona, 51, was considered one of the country's most accomplished labor lawyers and served as Labor Minister Antonio Bassolino's top aide. He was Transport Ministry undersecretary in the 1995-96 government of then-Premier Lamberto Dini. President Carlo Azeglio Ciampi and others went to the morgue to pay homage to D'Antona, and labor groups planned days of protests in cities across Italy. -=-=- 