The bizarre death of a top female chemist found drowned at a New Jersey water-treatment plant was ruled a homicide Monday — and police believe the culprit may be one of her 85 co-workers.

Geetha Angara (search), a well-liked mom of three, was "either pushed or shoved" through the opening of a 35-foot-deep tank, said Passaic County Prosecutor James Avigliano (search).

Angara, 43, was last seen alive a week ago as she walked to a remote part of the Passaic Valley Water Commission (search) plant in Totowa to calibrate a pump and take water samples.

When she didn't check out of the plant that night, and never returned home, authorities began a search.

They drained millions of gallons of water from the tanks at the plant, which processes 75 million gallons of drinking water a day, and found her body in one of them.

Next to her nearly-frozen corpse was her hand-held radio and clipboard, which looked as if they had been thrown in after her.

It's also not likely Angara could have lifted the heavy grating covering the tank herself, authorities said.

The reason one of her co-workers is suspected is because of the plant's high security, which tightly monitors the comings and goings of workers and visitors.

But while there are many security cameras around the plant, there were none in the area of the murder, police said.

Meanwhile, sheriff's deputies calmed jolted plant workers who were worried about having a coldblooded killer in their midst. They conducted armed patrols of the plant and told employees to stay in pairs at all times.

They plan to interview and reinterview every employee.

Angara's family was too distraught to talk last night and her neighbors, who had assumed her death was an accident, were stunned.

"My heart goes out to her husband. She was a great woman," said Rachel Allocco, 73, who lives next door. "She always checked up on me. If I was sick, she'd always call. And she was a hard worker. She'd leave the house at 5:30 a.m. and wouldn't get home until 6 at night."

Allocco said she spoke with Angara's heartsick husband, Jaya, at the funeral last weekend, which was also attended by the couple's two daughters, 19 and 8, and a son, 13.

"He said to me, 'Now I have to both be mother and father to them,'" Allocco said.

Indian-born Angara, a senior chemist, had worked at the plant for nearly 12 years.