An Italian magistrate has dropped his investigation of charges that a senior military intelligence agent coached Mehmet Ali Agca into claiming that Bulgarian agents had organized the plot to assassinate Pope John Paul II, according to judicial sources.

These sources said yesterday that Magistrate Franco Ionta found insufficient evidence that Gen. Pietro Musumeci, the former deputy director of the Italian military intelligence agency, had given Agca the information that led him to claim Bulgarian secret agents had been behind his 1981 effort to kill the pope.

Agca was convicted of wounding the pope and sentenced to life in prison. Later he claimed to be acting for a group of Turkish gangsters working for Bulgarians. His charges sparked a second trial.

Judge Ionta was acting apart from the court headed by Judge Severino Santiapichi that is trying Agca and his alleged accomplices. Ionta began his investigations after Naples underworld figure Giovanni Pandico, who had turned state's evidence in a mass trial of gangsters, said that while he was in the same jail as Agca in 1982, the Turk had been visited and coached by Gen. Musumeci. The general's former aide, Francesco Pazienza, was called the mastermind of the plan.

Musumeci is serving a nine-year jail term for his role in a neofascist terrorist bombing. Pazienza is being held in New York pending extradition to Italy on charges of involvement in financial scandals here.

Ionta's investigation apparently turned up no more on the issue than Judge Santiapichi's trial, which brought both Pandico and Musumeci to the witness stand with inconclusive results. Pandico and Agca, whose own often contradictory testimony remains the primary foundation for the papal plot trial, accused each other of lying.

Musumeci claimed that he had left the secret service in June 1981, almost a year before his alleged contacts with Agca in the jail.

Only one of three Bulgarians being tried by Santiapichi is in Italian custody.

Two indicted Bulgarian diplomats are to be interviewed in Sofia by Santiapichi Thursday. Defense officials estimate that the trial will close by February.