WASHINGTON (AP) _ The Reagan administration insisted Monday it had solid intelligence that Libya was behind the April 1986 bombing of a discotheque in West Berlin, despite fresh evidence from West Germany that Syria played an important role in the attack.

''It's clear enough from the evidence that's been made available that Libya was directly involved in that bombing,'' Secretary of State George P. Shultz said.

''The fact that somebody has a connection with another country that may also have been involved doesn't in any way lessen the Libyan involvement,'' Shultz said at a news conference in Ottawa, Canada.

In justifying the air raid against Libya 10 days after the bombing of the La Belle disco, President Reagan alluded to the interception of at least two messages from the Libyan People's Bureau in East Berlin to Tripoli.

Other U.S. officials said the evidence consisted of transmissions from the Libyan People's Bureau, or embassy, in East Berlin before the bombing and a confirmation of the explosion afterwards. The transcripts, however, have not been released to the public or to West German investigators.

The Libyan embassy changed its methods of communicating with Tripoli after Reagan alluded to the intercepts publicly, said one official, speaking on condition of anonymity.

The U.S. air raid against Tripoli and Benghazi killed dozens of Libyans, including the adopted daughter of Col. Moammar Gadhafi.

In their investigations of the disco bombing, West German officials at first focused on Libyan involvement, but later said they were unable to prove the connection.

On Monday, West German police arrested Christina Gabriele Endrigkeit, 27, who is accused of planting the bomb in the La Belle night club on April 5, 1986. Two U.S. soldiers died from the blast.

Phyllis Oakley, a State Department spokeswoman, said that ''while the information which led to the arrest of Christina Endrigkeit may suggest the involvement of another country, it in no way contradicts the evidence about Libya.''

The administration, she said ''will be following the Berlin investigation closely and with special interest in the possibility that another government, in addition to the government of Libya, may have been involved.''

The State Department spokeswoman did not mention Syria by name and declined to predict what action the administration might take if West German officials find more evidence of Syrian involvement.

Both Libya and Syria are on the State Department list of governments that support terrorism.

Endrigkeit was allegedly working with two convicted Palestinian terrorists, Ahmed Nawaf Hasi and his brother Nezar Hindawi, who allegedly have strong ties with Syrian intelligence.

Hasi was convicted on March 29, 1986, of bombing West Berlin's German-Arab Friendship Society and injuring nine people, and the West German court said the attack was plotted by Hindawi and Syrian intelligence officials.

Hasi, now serving a 14-year sentence, had drawings of the La Belle club when he was arrested and is considered a suspect in the case, said West German officials.

Hindawi was convicted by a British court in October 1986 for attempting to blow up an Israeli El Al airliner with 375 people aboard on April 17, 1986. He was sentenced to 45 years in prison.

British Prime Minister Margaret Thatcher severed diplomatic relations with Syria in retaliation its alleged involvement in that plot, a step that government and private experts in the United States say helped persuade Syria to curtail attacks by terrorists against targets in Europe.

West German officials say that the only link they have uncovered between Hasi and Libya are airline tickets to Libya found in his posession in 1985 and small payment by bank transfer he received from the Libyan Embassy in Bonn.

One U.S. official who deals with terrorist issues said ''Hasi may have been working both sides of the street, in effect double-billing his patrons,'' Libya and Syria.

''There is no evidence that Syria knew of Libya's role and vice versa,'' said the offical, speaking on condition of anonymity.

''If you are talking about a three-legged stool, you have Libyan involvement, which is proven, you have information which is suggestive of Syrian involvement, and a third leg on the stool remains unproven, that there was a connection between the two.''

Syria and Libya both played roles in the attacks on the Rome and Vienna airports on Dec. 27, 1985, the official said.

Testimony of the lone terrorist who survived the Rome incident was that team members were trained in the Syrian-occupied Bekaa Valley in Lebanon, then went to the Syrian capital of Damascus for further training, then travelled to Libya where they were issued weapons along with passports.

In the aftermath of the U.S. air raid on Libya, the late CIA director William J. Casey criticized U.S. news reports of the communications intercept.

However, Reagan alluded to the intercepts in his broadcast address on the night of the raid.

''On April 4, the people's bureau alerted Tripoli that the attack would be carried out the following morning,'' Reagan said in his address. ''The next day, they reported back to Tripoli on the great success of their mission.''

Other administration officials later supplied more details to cement the U.S. case against Gadhafi.