CHICAGO -- Three movie projectionists union activists were acquitted Friday of arson conspiracy charges related to a wave of smoke bomb attacks on theaters across the country during contract negotiations six years ago.

The union's secretary-treasurer, Albin Brenkus, while acquitted of the most serious charges, was found guilty of obstruction of justice for telling a co-defendant to lie to federal investigators.

The government's lead witness in the trial was longtime union activist Kent Dickinson of Bonfield, who testified that Brenkus showed him how to mix chlorine tablets with brake fluid to set off billowing clouds of smoke. He testified that Brenkus even supplied him with the chemicals.

Dickinson is one of four union members who pleaded guilty earlier and are awaiting sentencing.

The federal jury deliberated only a few hours before acquitting Brenkus and two co-defendants of conspiracy to commit arson, interstate travel to commit arson and related offenses.

The verdict was a blow to the government in a case that has drawn national attention.

Brenkus and his co-defendants were accused of being involved in the setting off of highly flammable smoke bombs in theaters operated by three big chains in 10 states, creating noxious smoke clouds and sending movie-goers fleeing in panic.

Prosecutors said it was a plot to pressure the chains to come to contract agreements with Motion Picture Projectionists and Video Technicians Local 110 of the International Alliance of Theatrical Stage Employees of the United States and Canada. Seven other defendants indicted in the case had earlier pleaded guilty.

Attorneys for Brenkus, 60, of suburban Willowbrook and the two co-defendants made no attempt to hide the fact that their clients had played rough with the three theater chains, AMC Entertainment Inc., Cinemark U.S.A. Inc. and Loews Cineplex Entertainment Corp.

Defense attorney Douglas Whitney even told jurors that Brenkus may have been involved in vandalism.

But the attorneys said that whatever Brenkus and his followers had done, it didn't add up to conspiracy to commit arson. Whitney said that when Brenkus sent his followers out to set off the smoke bombs he had no idea that the heat they generated could start fires.

Co-defendant Peter C. Lipa, 48, of Wonder Lake was acquitted of arson conspiracy and interstate travel to commit arson, and Gregory J. Tortorello, 26, who was in the Army at the time of his arrest, was acquitted of arson conspiracy and attempted arson.

Brenkus had been charged with arson conspiracy and a dozen other arson related counts, as well as obstruction of justice. He was convicted of telling another defendant in the case to play dumb if subpoenaed by a federal grand jury and say he "don't know nobody" and "don't know nothing."

U.S. District Judge Suzanne B. Conlon set sentencing for May 3. Brenkus faces a maximum sentence of 10 years in federal prison.

According to prosecutors, theaters were attacked in Illinois, Indiana, Iowa, Kansas, Kentucky, New Jersey, New York, Ohio, Texas and Wisconsin.