WASHINGTON, June 25 (Xinhua) -- Local authorities said Monday they are investigating what caused over two dozen people working in an e-cigarette plant in Salem, Massachusetts, to fall ill.

Twenty-nine people were taken to hospital Sunday night, with many of them suffering breathing problems and nausea, acting Salem Fire Chief Gerry Giunta told CBS News on Monday.

One of them suffered an apparent seizure and was rushed to a Boston hospital. The majority were treated and released but some remained hospitalized, Giunta said.

There seemed to be "panic" in the building as workers began running out on Sunday night, Salem Deputy Fire Chief Alan Dionne said earlier, calling it "mass hysteria."

Some 450 to 500 people were working at the time of the incident, and people started reporting symptoms not long after a shift change, according to the CBS News report.

A chlorine leak had been reported in the building earlier. However, Jennifer Mieth, a spokesperson for the state's fire services, said the crew on the ground did not believe Sunday's incident was connected to the leak.