PASSAIC COUNTY-Something wasn't right at the Passaic Valley water plant on the evening of Feb. 8. Geetha Angara, a chemist who monitored the quality of drinking water in colossal tanks, was not at her work station, but her belongings were. Her car was where she left it in the parking lot. She was supposed to have arrived home hours earlier, but her family had not seen her, and she hadn't called to say she would be working late. Most significantly, a heavy metal grate covering a million-gallon water tank was slightly askew; screws that had secured it were either gone or broken. It was only after searches with helicopters, cadaver-sniffing dogs and scuba divers that authorities confirmed what they had started to fear hours earlier: Angara was dead at the bottom of one of the tanks. What came next was an even greater shocker. Authorities pronounced her death a murder and said they believe she was killed by a co-worker. But nearly four months later, authorities have yet to zero in on a suspect, and they are frustrated. ``You watch the TV shows and there's a motive and a confession,'' said prosecutor James Avigliano. ``Here, we have no obvious motive, no confession and no physical evidence.'' Angara, 43, was last seen working in the laboratory, where her duties included calibrating water clarity sensors. At some point, she was attacked and rendered unconscious; prosecutors will not say whether she was struck with fists or an object, or whether she was choked. That is information only the killer would know for sure, and it has been withheld from Angara's family as well as the public. Her attacker dropped her into the water tank through a four-foot opening that is usually covered by a 50-pound metal panel, secured by about a dozen screws. About 24 hours after she was reported missing, her body was found in a different tank, having floated through pipes from the tank into which she was dumped. She was alive when her body hit the frigid water; an autopsy listed the cause of death as drowning. From there, the trail has gone cold, even though authorities say it is a virtual certainty that Angara was killed by a co-worker. There is only one way into and out of the plant, through a 10-foot-high gate monitored by a video camera, past a security guard. Investigators have reviewed videotapes that show everyone who came or went. Each of the 50 people who were at work that day has been interviewed, along with 30 others. ``We've narrowed it down to eight people we're still looking at,'' Avigliano said. ``It's very frustrating, especially for the detectives working the case. They can't stand it when somebody gets over on them.'' Angara had recently been promoted to senior chemist, but investigators discount jealousy among co-workers as a potential motive. Many of Angara's co-workers refused to discuss the case, citing the investigation. ``She was a nice lady, very friendly to everybody,'' said Indrakumar Patel, one of Angara's co-workers. ``That's all I can say.'' Angara was born in Madras, India, earning her master's degree at Loyola College in India, where she was the first female student. She graduated with the gold medal, given to the top student in her class, and came to the United States in 1984. She, her husband and their three children, ages 9 to 20, lived in Holmdel. The prosecutor said the best chance for a break in the case will be ``if someone gets an attack of conscience and confesses, or if someone who knows something comes forward.'' ``So far,'' he said, ``neither of those things has happened.''