Two members of a Philadelphia Ironworkers union admitted roles in 13 acts of sabotage and intimidation Monday on behalf of a group that prosecutors say routinely damaged school, business, and even church construction sites to pressure builders into hiring union workers.

Francis Sean O'Donnell and William Gillin, both 43, each pleaded guilty to racketeering conspiracy and arson during a hearing before U.S. District Judge Michael M. Baylson. Theirs were the first of eight pleas expected from members of Ironworkers Local 401 during the next two weeks.

"I am guilty of all of the charges presented by the government," Gillin said moments after confessing to a list of crimes including the December 2012 arson at a Quaker meetinghouse under construction in Chestnut Hill.

That attack prompted louder calls from area builders to crack down on what they described as Philadelphia's entrenched culture of union intimidation and violent retribution.

Both O'Donnell and Gillin face potential sentences of decades in prison at hearings scheduled for January.

Prosecutors would not say Monday whether either man had agreed to testify against the union's top leaders, including its longtime head, Joseph Dougherty, who is scheduled to take his case to trial in January.

But in court filings, they described the union Dougherty ran as one in which violence served as more than just a negotiating tool - it was ingrained into the structure of the organization.

Dubbed "nightwork" by members, arsons, vandalism, and beatings delivered on picket lines were meant to cow contractors who hired nonunion labor, Assistant U.S. Attorney Robert Livermore wrote.

Ironworkers who took on such off-duty work for the union were rewarded with plum assignments during regular working hours, and members ran for leadership positions based on the acts of sabotage they could claim on their resumés, according to the filings.

"Older or retired ironworkers often regaled younger ironworkers with stories of fisticuffs, tearing down buildings erected by nonunion members or damaging equipment belonging to nonunion contractors," Livermore wrote. "The most violent or more destructive ironworkers became legends within the union."

For O'Donnell, who joined the local in 1989 two weeks after graduating from high school, that mind-set was part of his family history.

His father was a member of the local. His uncle, now a codefendant, did a stint as an elected business agent charged with drumming up jobs for union members.

In 2011, O'Donnell attained a similar post, and was put in charge of looking for work at construction sites in portions of Delaware and Chester Counties.

Like all the union's business agents, he was expected to negotiate jobs for members. His negotiating tool of choice, prosecutors said, was an eight-pound sledgehammer.

O'Donnell admitted Monday that from 2009 to 2013, he either ordered or participated in 11 attacks on sites that included future elementary schools, firehouses, and shopping centers.

He kept detailed notes on each incident and later read them aloud at the union's executive meetings.

In one, seized by the FBI in a raid on the union's offices this year, O'Donnell reported that the contractor building a school in Sharon Hill had "run into another anchor bolt problem . . . thanks again to the Shadow Gang."

Asked Monday why he was pleading guilty, O'Donnell responded, "To show my kids to do the right thing. There's nothing wrong with being honest."

Gillin's case, prosecutors said, demonstrates that willingness to break the law was also required of the union's rank and file.

Facing economic pressures at home, Gillin began to notice that only union members who participated in nightwork were selected by the leadership for long-term jobs with opportunities for overtime, his plea documents said.

That realization led him first to the Ironworkers picket line outside the construction site of the Goldtex Apartments, at 12th and Wood Streets, in the summer of 2012. He and his fellow members blocked contractors' supply trucks, slashed tires, and laid spike strips across the site's entryways in hope of halting the nonunion project.

Gillin received a similar call for his services when a nonunion contractor secured a job building a new meetinghouse in Chestnut Hill. On the night of Dec. 20, 2012, he and two fellow union members broke into the site under the cover of a rainstorm.

Gillin poured gasoline over the construction crew's crane and set it ablaze, while his accomplices cut the building's metal infrastructure with an acetylene torch.

Congratulating each other on the attack two days later, Gillin received a text from an accomplice that was later detailed in court filings.

"T.H.U.G . . . The Helpful Union Guys," it read.

Gillin responded: "We should shut r phones off."

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