Sen. Roberto Ruffilli, a close friend and adviser of Premier Ciriaco De Mita, was shot to death Saturday at his home in northern Italy. An anonymous caller said Red Brigades terrorists were responsible.

Investigators told Italian news agencies the body of the 51-year-old Christian Democrat senator was found on a couch in his living room at Forli, 40 miles southeast of Bologna. They said he was shot in the back of the neck.A spokesman for the Interior Ministry said an anonymous telephone caller to the Bologna office of the Rome newspaper La Repubblica asserted responsibility on behalf of the Red Brigades.

The spokesman quoted the caller as saying, "We have killed Senator Ruffilli, attacking at the heart of the state. Red Brigades for the Formation of the Fighting Communist Party."

President Francesco Cossiga said: "Our institutions and all the forces that are alive in the country will know how to act with firmness for the defense of liberty and of right.

"The assassination of Sen. Roberto Ruffilli is a horrendous crime against the most sacred of human values life. It is an abominable attack on the parliamentary and representative institutions which are the fulcrum of our democratic state."

Maurizio Laudi, a magistrate who has investigated the terrorist group, was quoted by the news agency AGI as saying the Red Brigades for the Formation of the Fighting Communist Party is the most dangerous and active group in what remains of the Red Brigades. He said it also is the most difficult to penetrate.

AGI said Ruffilli had received threatening phone calls in recent days.

He led the Christian Democrat committee on problems of the state and was working with the premier on institutional reform. Ruffilli also was on the parliamentary commission that investigated the P-2 Masonic Lodge scandal.

Premier De Mita convened a special meeting Saturday night of Interior Minister Antonio Gava and the chiefs of the Italy's police forces.

"The criminal warning of the terrorists could not have been any clearer," he said. "It has the same clarity with which 10 years ago through the person of Aldo Moro they aimed at his political program."

Red Brigades terrorists kidnapped and killed Moro, a former premier, in 1978.

According to the news agency ANSA, investigators at Ruffilli's home found the door open and no signs of forced entry. State-run RAI television said the assassins fled on foot after the shooting at about 1 p.m.

In a dispatch quoting government sources it did not identify, AGI said the car bombing that killed five people and wounded 17 at a Naples club for U.S. military personnel Thursday night may have inspired terrorists to kill Rufilli. An American servicewoman and four Italians were killed in the blast.

The murder of air force Gen. Licio Giorgieri in Rome on March 20, 1987, was the last terrorist act for which a branch of the Red Brigades claimed responsibility.

A month before that, the Red Brigades for the Formation of Fighting Communist Party claimed responsibility for an armored car robbery in Rome.

In December 1981, Red Brigade terrorists disguised as plumbers kidnapped U.S. Brig. Gen. James Dozier, who was with the NATO command in Italy, from his home in Verona. He was rescued 42 days later by a special police unit in Padua.

In February 1984, terrorists in Rome fatally shot Leamon Hunt, a U.S. diplomat and director-general of the Multinational Force and Observers in Sinai. The Red Brigades claimed responsibility.