The credibility of the pope's would-be assassin as a reliable witness has been questioned by magistrates who have dealt with him in other criminal cases, according to Italian and Turkish court documents obtained by The Washington Post.

The documents show that a total of seven people specifically accused by Mehmet Ali Agca of crimes ranging from drug smuggling to conspiracy to murder have already been absolved either for "lack of proof" relating to cases in which Agca was a key accuser, or because Agca can be shown to have lied.

The strongest challenge to the credibility of the prosecution's star witness in the papal conspiracy case has come from a public prosecutor in the northern Italian city of Trento investigating a arms- and drug-smuggling network between Europe and the Middle East.

A report by Trento prosecutor Enrico Cavalieri, dated April 12, 1984, maintains that Agca was "caught lying" while giving evidence against one of the key suspects in the papal case, Turkish businessman Bekir Celenk, on drug-smuggling charges.

At least some of Agca's information about Celenk's alleged involvement with drug smugglers, the prosecutor writes, appeared to have come from the press.

Asserting that Agca's testimony has been "repudiated by the sensational lies during the last part of his interrogation," prosecutor Cavalieri ordered that drug-smuggling charges against Celenk be dropped. He added, however, that Celenk should still stand trial for arms smuggling on the basis of testimony provided by witnesses other than Agca.

Cavalieri also dropped drug smuggling- charges against three other Turks directly accused by Agca: Saral Atalay, Mehmet Cantas, and Ismail Oflu. (Like Celenk, Cantas and Oflu will stand trial for arms smuggling.)

The prosecutor in the papal case, Antonio Albano, has acknowledged that Agca has lied or been mistaken on two specific occasions since beginning to give testimony against his alleged Bulgarian co-conspirators. But he argues that the pope's would-be assassin has on balance "earned the right to be believed" by demonstrating personal knowledge of his alleged accomplices and describing a plausible escape plan involving a diplomatically sealed truck.

In Turkey, meanwhile, a report by a military prosecutor dated Feb. 16, 1984 has ordered that charges be dropped against three people accused by Agca of involvement in the February 1979 murder of newspaper editor Abdi Ipekci. Turkish magistrates traveled to Italy in mid-June 1983 to question Agca on the Ipekci case. The three are: Mustafa Kemal Derinkok, a prominent Istanbul businessman. Agca alleged that Derinkok was involved in the planning of Ipekci's murder because he wanted to remove a potential barrier to purchase of the newspaper Milliyet. According to the prosecutor, "no evidence against Derinkok was found apart from Agca's allegations and it was decided to drop the case." Gungor Uygor, a lawyer who Agca said provided a hide-out for one of his Gray Wolf associates. Agca's allegations could not be supported and the case was dropped. Yilmaz Yuksel, a prison guard accused by Agca of helping him escape from the top-security Kartal-Maltepe prison outside Istanbul in return for a bribe. An investigation established that Yuksel had been transferred from the prison the month before Agca's escape and therefore could not have been involved.

In his most recent testimony to Turkish magistrates, Agca insisted that he had not taken part directly in Ipekci's assassination. The real killers, he now claimed, were his Gray Wolf friends: Oral Celik and Yalcin Ozbey.

There is one other Italian investigation involving Agca as a witness: an inquiry into an alleged Bulgarian spying ring in Rome. The investigation is continuing and no formal indictments have yet been made.