A court in India has found seven people guilty of attacking the US cultural centre in Calcutta in January 2002.
Ansari, the attack mastermind, was arrested in Dubai
Those convicted include Aftab Ahmed Ansari, who the judge said had planned the attack in which five policemen were killed and nearly 20 others injured.
Sentencing is due on Wednesday. Two men were acquitted for lack of evidence.
The attack heightened tensions in South Asia, coming just weeks after a bloody raid on parliament in Delhi. Pakistan denied links to either incident.
Tuesday's verdict came after 300 court hearings involving 123 prosecution witnesses and three defence witnesses over the last three years.
Ansari and the six others convicted of the attack now face a possible death penalty.
Their defence lawyer, Syed Shahid Imam, said they would appeal against the verdict in a higher court after sentencing had been announced.
Tight security at the US Calcutta consulate after the attack
The two acquitted, Deepak Kumar Patel and Shakeel Malik, were cleared of all charges.
The incident happened on 22 January 2002 when four men, draped in shawls, sped up to the American Centre building in Calcutta on two motorcycles, refusing to stop at checkpoints and shooting at police guards who returned fire.
Four of the dead officers were killed on the spot. They were all from the Calcutta police or a private security agency. The consular staff escaped unhurt.
Ansari was later arrested in Dubai and extradited to India.
Judge Basudev Majumdar, who headed the special court that tried Ansari and the other accused, said the men had been found guilty of "waging war against the legally constituted government of India", murder and several other offences.
The special court was set up inside a jail in Calcutta after intelligence reports suggested that Ansari might try to escape while on his way to the court.
India linked two Pakistani men to the Calcutta bloodshed. Pakistan dismissed the allegation as a "joke".
The Indian police said the men were killed in a gun battle in the northern state of Jharkhand, but that one of them had confessed to the Calcutta attack shortly before he died.
Islamabad also denied any role in the dramatic suicide raid in December 2001 by militants on India's parliament.
That incident triggered a military confrontation which saw the nuclear-armed rivals mobilise hundreds of thousands of troops, sparking fears of all-out conflict.