MOSCOW (AP) _ Prosecutors investigating the August coup reiterated today that Mikhail Gorbachev was not involved, but said the plotters' association with him could have led them to believe he would eventually take their side.

Some people have suggested the former Soviet president might have staged the coup in order to purge the Kremlin of his hard-line enemies or to stir up public support for his cause.

But Yevgeny Lisov, head of the investigation team, said at a news conference today that Gorbachev either ''directly or indirectly'' let the eight-man group that led the coup know he would not support them.

His comments came a few days after lawyers for the officials accused of masterminding the coup demanded that Gorbachev be barred from leaving the country because he is a potential defense witness. On Jan. 14, prosecutors dropped high treason charges against 12 former high-ranking Soviet officials involved in the three-day coup, but said they could still face the death penalty for ''conspiring to seize power.''

Gorbachev testified in September before Russian Chief Prosecutor Valentin Stepankov, who said the president had not been involved in planning the coup. He also said Gorbachev had been unaware of the preparations that had taken place behind his back.

Lisov said today that view had not changed.

''There is no doubt about it,'' Lisov said, ''although his long association with the coup plotters and some specifics of his character gave the plotters the right to think that sooner or later they would be able to attract Gorbachev to their side.''

''Eventually this factor was the decisive one that compelled them to try to seize power,'' he said without giving any specifics.

The conspiracy charges filed against the 12 former officials resulted from a 4 1/2 -month investigation by the Russian prosecutor's office, in which 125 volumes of evidence were collected.

Fifteen people are under investigation, but three have not been charged due to poor health. Those being investigated could face a maximum penalty of death by firing squad if found guilty.

The accused coup leaders include former KGB chief Vladimir Kryuchkov, Prime Minister Valentin Pavlov, Defense Minister Dmitri Yazov, Vice President Gennady Yanayev, Alexander Tizyakov, head of the state enterprise association, and Vasiliy Starodubtsev, leader of Peasants Union.

Stepankov has said they should be tried in public, but no trial date has been set. He says the defendants need time to familiarize themselves with the materials in the case.