State and federal authorities are investigating the deaths of dozens of federally protected barn swallows outside a water filtration plant in South Shore, and an employee who found the birds said they appeared to have been intentionally killed.

The worker, who asked not to be named for fear of reprisals, told the Sun-Times that another employee discovered the dislodged birds’ nests in the early morning hours of June 28 around the chlorine building at the Eugene Sawyer Water Purification Plant at 3300 E. Cheltenham Place near Rainbow Beach.

The source said he and the other worker then went around to the building’s four entrances and discovered 15 living barn swallows from the roughly 40 adult birds and hatchlings that were found dead amid the downed nests.

Photos provided to the Sun-Times show the dead birds after they were lined up on a curb by the employees that found them.

“Rather than there just being a clump of mud directly on the ground” where the nests were found, the ground “had swipe marks across it as if had been stepped on,” according to the source, who said it appeared as if the nests had been smashed into the pavement.

“I couldn’t believe it. That just made me sick,” he added. “They had families, babies and eggs.”

Barn swallows are among the hundreds of birds protected under the Migratory Bird Treaty Act of 1918, a federal law that makes it illegal to kill, possess, import, export, transport, sell, purchase or barter protected species without a permit. Those who violate the law can face a misdemeanor charge that carries a maximum prison sentence of six months and fines of up to $15,000.

Representatives for the Illinois Department of Natural Resources and the U.S. Fish and Wildlife Service both confirmed their agencies were probing the incident but said the investigations were still in their preliminary stages. IDNR spokesman Tim Schweizer said an officer with the agency has visited the plant but still needs to interview more people who were involved.

Megan Vidis, a spokeswoman for the Department of Water Management, said officials in the department are “aware of the incident” and have been cooperating with federal investigators.

Although access to the plant is restricted and the incident happened when few employees were working, Vidis said no action had been taken against anyone at this point.

“If deemed appropriate, the proper disciplinary actions will be taken,” she said.

The Office of the Solicitor for the U.S. Department of Interior issued an opinion in 2017 stating that the Migratory Bird Treaty Act applies to intentional acts but doesn’t prohibit the incidental or accidental killing of birds. However, Ken Adams, a criminal investigator for the U.S. Fish and Wildlife Service, said it’s “too early into this investigation to throw too many opinions out there or guess what’s going on.”

The man who reported the dead barn swallows to authorities said he initially told a supervisor about the incident when he was leaving work the morning of June 28.

“In my eyes, somebody committed a federal crime. It’s my duty and my responsibility to alert the authorities,” said the source, who also informed state officials.

Since discovering the dead birds, he said he hasn’t been able to sleep.

“This really got to me,” he said of the incident.

Officials with International Union of Operating Engineers Local 399, which represents workers at the plant, did not immediately respond to a request for comment.