PROVIDENCE, R.I. >> A carabiner is the only piece of equipment investigators have found that failed in a circus accident in which eight aerial acrobats plummeted to the ground, a public safety official said Monday.

Providence Public Safety Commissioner Steven Pare stopped short of saying the broken carabiner was the cause of Sunday’s accident at the Ringling Bros. and Barnum and Bailey circus. He said federal workplace safety investigators were trying to determine why it snapped.

“We don’t know if it was metal fatigue, if it wasn’t properly positioned or something else,” he added. “We just don’t know.”

The 4- to 5-inch steel clip was one of several pieces at the top of a chandelier-like apparatus that suspended the performers in the air, fire officials said at a news conference. The clip was found in three pieces on the ground with its spine snapped.

Two of the acrobats were in critical condition Monday. Family members say their injuries included a pierced liver and neck and back fractures. Stephen Payne, a spokesman for Feld Entertainment, the parent company of Ringling Bros., said none of the injuries appear to be life-threatening.

“We are hopeful that all of these performers will achieve a full recovery and be able to return to the show at some point,” Payne said. A dancer who was on the ground was also injured and released from the hospital Sunday, he said.

The circus said the women are from the United States, Brazil, Bulgaria and Ukraine.

The father of one of the women, Widny Neves, said his daughter broke her right arm and suffered back and neck fractures. Roitner Neves said Widny, who had been traveling with the circus for more than four years, was the in the center of the apparatus and upside-down when it fell.

“It was like a plunge into darkness,” he said.

She is 25 and from Joinville, Brazil, where her family owns a circus academy.

“In this profession, you run the risk of being injured,” he said. “It’s like being a race car driver or a gymnast. There’s always the risk.”

Two women were listed in critical condition by Rhode Island Hospital, Dayana Costa and Julissa Segrera. Costa’s cousin told Brazil’s TV Globo’s G1 internet news portal that she had undergone surgery in her back.

Another injured acrobat, Stefany Neves, fractured both her ankles and had her liver pierced by her ribs, her sister Renata Neves told the same news outlet. She was listed in serious condition.

Investigators from the federal Occupational Safety and Health Administration were leading the investigation.

The accident at the Dunkin’ Donuts Center was witnessed by an audience of about 3,900, many of them children.

The performers — called “hairialists” by the circus — hang “like a human chandelier” using their hair during the act. The performance is supposed to include choreography as well as spinning, hanging from hoops and rolling down wrapped silks while suspended 25 to 40 feet up.

Video taken by audience members shows a curtain dropping to reveal the eight women hanging from a metal apparatus suspended from above. Seconds later, as they begin to perform, the women fall, and the apparatus lands on them.

The hair-hanging stunt is described on the circus’ website as being the brainchild of husband-and-wife team Andrey and Viktoriya Medeiros. Viktoriya Medeiros is among the injured.

Payne said the equipment has been used dozens of times per week since the beginning of the year, and that a circus crew had installed it last week. The crew also inspects it, he said, and performers generally check their own rigging.

Paul Doughty, of the Providence Fire Department, said on Monday that circus officials told investigators they had done a visual inspection of the carabiner before the show. He said the carabiner, which was hooked into various other pieces of equipment, could have had a weakness that was not apparent. He did not know how old it was or how many times it had been used.

State and city officials have no role in inspecting such equipment, they said.

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Associated Press writers Adriana Gomez Licon from Sao Paulo, Pat Eaton-Robb in Hartford, Conn., and Jessica Gresko in Washington contributed to this report.