By Alli Knothe Globe Correspondent

A Brockton teenager is facing charges after he allegedly robbed an elderly woman, threatened a witness with a knife, and bit a nurse when he was taken to the hospital for evaluation, police said.

Evyn Garner, 19, faces charges including armed robbery while masked, assault and battery, assault with a dangerous weapon, and unlicensed operation of a motor vehicle, said Lieutenant Robert Sergio.

Garner allegedly approached an elderly woman from behind on Stevens Avenue at about 8:30 a.m. Wednesday, wearing a black mask. Witnesses reported the suspect grabbed the woman around the neck and took her purse before running away.

A man who witnessed the attack then ran after the suspect. The witness backed off, however, when the suspect threatened him with a knife, police said.

An officer working a detail nearby saw a man running with a ski mask on his face and alerted other officers.

“A guy is masked and it’s 90 degrees out -- something’s happened,” Sergio said.

The suspect got into a vehicle he had parked near the crime scene and drove away. Another Brockton police officer saw the vehicle matching the description and pulled the driver over. Police located a large serrated kitchen knife and well as a brown purse with the victim’s belongings in the vehicle.

Garner was later transported to Brockton Hospital for evaluation, where he attacked and bit a nurse, police said. He was later released and taken into custody.

Sergio said Garner was wanted for two outstanding warrants.