A homeless man who told police he regularly slept at Bellevue Hospital and masqueraded as an employee was arrested on charges he raped and killed a pregnant pathologist at the historic facility.

The swift arrest Monday of Steven Smith, 23, culminated an intense investigation that began Sunday to solve the crime at the nation's oldest hospital. Mayor Edward Koch had declared the brutal killing the city's No. 1 police priority.Smith, who previously was arrested on charges of petty theft at East Side hospital, was to be arraigned Tuesday on charges of second-degree murder, first-degree rape and first-degree robbery for the death of Dr. Kathryn Hinnant, 33.

He was arrested Monday in the lunchroom of a shelter for homeless men on Ward's Island in the East River after three men there told police Smith was trying to sell some of the doctor's credit cards and articles of clothing.

Smith told police a second man helped him carry out the rape and killing at the hospital, but Police Commissioner Benjamin Ward said police "have not even confirmed that this person exists."

Nevertheless, he said the team of 50 homicide detectives at the rambling facility just blocks from the United Nations along the East River will remain "until we are convinced this case is closed."

Hinnant, who was five months pregnant, was strangled, raped and robbed of her money and her mink coat by an assailant who attacked her in a secluded fourth-floor office sometime Saturday night or early Sunday.

Ward indicated she might have been strangled with an electric cord.

Police at first thought Hinnant had been beaten to death.