A cashier working alone in a convenience store was stabbed to death as police, tipped off to a possible holdup, watched the business.

Relatives of Melissa Roy were outraged over her slaying Thursday, contending police should have been guarding her inside the store rather than watching from outside."They let a 20-year-old girl get stabbed to death," said Roy's mother, Carol Roy, who spoke with the part-time college student shortly before her slaying.

"Why would they leave her there? Don't tell me it was of her own free will. . . . They made her feel - she told me on the phone - `It's no big deal,' " Mrs. Roy said.

Police arrested Henry L. Guay, 25, of Lewiston, on a murder charge after following his car from the store, called The Big Apple. A weapon was recovered. Guay was held without bail in Androscoggin County Jail in Auburn pending a hearing Friday.

Police Chief Laurent Gilbert said officers, acting on an anonymous tip about a possible robbery attempt, had the store under surveillance and supplied Roy with a special alarm that would have alerted patrol cars.

The store manager was told of the tip but decided against closing for the evening, Gilbert said.

Police did not station an officer inside the store because that could have led to a hostage situation, Gilbert said.