PROVIDENCE, R.I. – The owners of a nightclub where a 2003 fire killed 100 people will plead no contest to involuntary manslaughter charges, and only one will have to serve prison time, their lawyer said yesterday. Victims’ relatives were outraged.

Kathleen Hagerty said Jeffrey and Michael Derderian will enter the pleas more than 3 1/2 years after pyrotechnics ignited foam soundproofing as a 1980s heavy metal band started playing at The Station nightclub.

Hagerty confirmed Michael Derderian will serve 4 years in a minimum security prison, with eligibility for a work release program, and that Jeffrey Derderian will receive a suspended 10-year sentence.

Relatives were furious about what they considered to be light punishments for the brothers’ role in the fourth-deadliest fire in U.S. history, a tragedy that touched untold thousands of people in the nation’s smallest state.

“I can’t believe the attorney general is just going to stand by and say OK to this,” said Diane Mattera, whose 29-year-old daughter, Tammy Mattera-Housa, died in the fire.

Hagerty confirmed the pleas after WJAR-TV and The Providence Journal reported on a letter Attorney General Patrick Lynch wrote to families of those killed to announce the plea deal. A spokesman for Lynch did not immediately return phone messages seeking comment; media reported that Lynch was making calls to family members yesterday night.

Lynch says in the letter that he objects to the sentences that Superior Court Judge Francis Darigan has said he will give the Derderians.

“Most significantly, I strongly disagree with the Court’s intention to sentence Jeffrey Derderian to less than jail,” he wrote. He added, however, that the plea deals mean the brothers are accepting criminal responsibility “despite months of denials.”

The Derderians will change their pleas on Sept. 29 and could be sentenced that day, according to the letter.

Many relatives of victims, including Robert Bruyere, whose stepdaughter, Bonnie Hamelin, died in the fire, said they learned about the plea from news reports.

Lynch “better hope I don’t see him in person, because I’ll be in jail,” Bruyere said in a telephone interview as his wife, Claire, sobbed in the background.

The plea comes as jury selection was under way for Michael Derderian’s criminal trial; his brother’s trial was to have followed.

“All I can say is that Jeffrey and Michael Derderian are looking to put a resolution to this,” Hagerty said.