Pinar Selek, 35, known for her critical studies of the Kurdish conflict in Turkey and her work with street children, was accused of helping separatist Kurdish rebels to organize the powerful blast at the entrance of the historic Space Bazaar in Istanbul on July 9, 1998.

The court said it was impossible to reach a conclusion on the cause of the blast, and that there was no other evidence to prove Selek was guilty, Anatolia reported.

She faced a life sentence if found guilty of involvement in the explosion, which claimed seven lives and left 127 others injured.

Selek was implicated in the case after she refused to give police the names of Kurdish rebels she had interviewed for a book she was writing. She was reportedly tortured in detention.

The court also dropped a separate charge of "aiding and abetting an illegal organization" against the defendant, citing the statute of limitations.

Selek had spent more than two years in jail when she was released in December 2000 following an initial expertise report that the blast was caused by a leak from a gas cannister in a nearby snack kiosk.

Three alleged militants from the rebel Kurdistan Workers' Party (PKK), on trial in the same case, were jailed for life for separatism, but their convictions stemmed from offenses unrelated to the blast, Anatolia reported.

Two others were sentenced to 12.5 years in jail, while a sixth defendant got five months in prison, the agency added.

Selek was a year into her master's studies when she was arrested. After her release from prison, she completed her thesis and received her master's degree.

She is now working with civic movements promoting women's rights and anti-war campaigns.

The Kurdish conflict in Turkey has claimed more than 37,000 lives since 1984 when the PKK, blacklisted as a terrorist group by Ankara, took up arms for self-rule in the mainly Kurdish southeast.

The group attacks mostly military targets, but has also carried bomb attacks on civilians.