PHILADELPHIA -- It was several weeks ago when vandals struck the building site, setting fire to a crane, cutting deep gashes in steel columns and loosening the bolts that anchored them. The episode resulted in about $500,000 in damage and was, the authorities believe, apparently an attempt to halt the construction of a Quaker meeting house.
The $5.8 million building was being constructed for the Chestnut Hill Friends, a Quaker community in a northern section of Philadelphia. Cuts in the steel columns that make up the building's frame were made with an acetylene torch, indicating that the attack was carried out by someone with both the equipment and the expertise to operate it, the police say, suggesting that it may have been the work of trade unionists who were disgruntled after being refused work on the site.
"It's not being done by 12-year-old vandals," said Lt. George McClay of the Philadelphia Police northwest detectives division, which is leading an investigation into the episode. Fire officials are treating the attack on the crane as arson.
Asked whether the damage might have been an attempt at intimidation by union members, Lieutenant McClay said: "It does point in that direction. Can I prove it? Absolutely not."
Pat Gillespie, business manager of the Philadelphia Building and Construction Trades Council, which represents about 60,000 construction workers in 42 local unions, rejected suggestions that members were responsible for vandalizing the meeting house. He said that the relatively small project would not generate enough union work to explain any such attack, and that union members would not want to jeopardize chances of future work by alienating potential employers.
The episode may have been a result of a payment dispute, he said. "Maybe that person had business dealings that didn't go too well."
Four union members visited the site during the week before the overnight attack on Dec. 20-21, asking questions about who was doing specific construction tasks, said Robert N. Reeves Jr., the president of E. Allen Reeves Inc., the contractor. The company operates an "open shop" that does not employ union members but may work with unionized subcontractors, Mr. Reeves said.
Each of the union members left the site after being told by employees that they could not give out information about the construction, Mr. Reeves said. The fourth told the site manager on Dec. 17 that "I'll do what I have to do," said the manager, who spoke on the condition of anonymity.
The vandalism has led the eastern Pennsylvania chapter of the Associated Builders and Contractors, a trade association, to offer a $50,000 reward to anyone whose information leads to the arrest and conviction of those responsible.
The episode is the latest in Philadelphia to prompt allegations that trade unions engage in violence and intimidation in an effort to secure work for their members.
In mid-December, an employee of Post Brothers Apartments, a local developer, was attacked with a crowbar by a member of the Ironworkers Union, according to Michael Pestronk, chief executive of the company. Post Brothers is in a dispute with local unions over its refusal to hire all-union labor to work on an apartment building in central Philadelphia
Mr. Gillespie said he was not aware that any union member had used a crowbar to attack Post Brothers employees, and he accused Mr. Pestronk of "exaggerating" the confrontations that have occurred at the site. "There have been some pushing and shoving instances that have happened on that site that I'm aware of," Mr. Gillespie said. "If anyone gets assaulted with a crowbar, that's a serious charge, a felony offense."
Post Brothers initially wanted to hire 40 percent union labor on the project, converting a factory into 163 loft apartments, but is using all nonunion workers after unions withdrew their members in protest at the company's position, Mr. Pestronk said.
"They told me, 'If you don't make this project 100 percent union, we are going to do everything in our power to stop this job,'" Mr. Pestronk said in an interview. Picketing of the Goldtex site began in December 2011 and employees have been harassed and attacked in episodes that have been captured on video and posted on a company Web site, he said.
Michael Resnick, Philadelphia's director of public safety, said that if the authorities were presented with evidence that union protesters were committing crimes, the offenders would be arrested and charged, as they have been in "two or three cases" during the Post Brothers protest.
Mr. Resnick rejected accusations by Mr. Pestronk and others in the construction industry that city authorities are lax in cases involving union harassment and intimidation. He said unions had a constitutionally protected right to protest against working practices, and that developers like Post Brothers must recognize that.
"People have a right to express their views," Mr. Resnick said in an interview.
Meg Mitchell, clerk of Chestnut Hill Friends, said the contractor for the meeting house project was selected on the basis of price, quality and its track record of building houses of worship. She said the community, which like all Quakers advocates nonviolence, was aware that the winning contractor was not unionized.
The new meeting house is expected to open on schedule in early summer now that the damage has been repaired, Ms. Mitchell wrote in an e-mail.
This article originally appeared in The New York Times.