The Tokyo District Court ended a 15-year trial Thursday by acquitting three men charged with firing mortar-launched projectiles in 1986 at the U.S. Yokota Air Base and the State Guesthouse in Tokyo during a meeting of leaders of developed nations.

Taketoshi Suga, 59, Hirofumi Sogame, 60, and Hiroshi Itagaki, 60, were charged in November 1987 with manufacturing mortar rounds in conspiracy with other activists.

Presiding Judge Nobuyuki Kiguchi said Thursday that while the defendants had probably manufactured other explosive devices, there was room for reasonable doubt as to their involvement in the attacks.

"There is no evidence that they created the mortar shells used in the case, and there is no proof that they conspired with those who actually fired them," he said.

The Tokyo District Public Prosecutor's Office said it will appeal the ruling. The first hearing was held in September 1988. It had been the longest ongoing trial at the district court.

Prosecutors said the three conspired with others and fired five mortar shells at the guesthouse in central Tokyo on May 4, 1986, to disrupt the talks, as well as launching five shells at the air base in western Tokyo on April 15 the same year. They were freed on bail in December 2002.