Two students were robbed at knifepoint Sunday, one in broad daylight, as Yale police searched for a single suspect whom they believe was responsible for both incidents. New Haven police were also investigating a third mugging at another location near campus on Sunday.

The first robbery occurred on Science Hill at 2:45 p.m. Sunday, when a man wielding a serrated kitchen knife allegedly threatened a Yale student who was walking up the pathway to Kline Biology Tower, said Yale Police Chief James Perrotti. The man then took the student’s money and headed down Hillhouse Avenue, said New Haven Police Sgt. Rebecca Sweeney.

A second Yale student was robbed on York Street, near Broadway, at about 11:30 p.m., Perrotti said. The suspect, described by Yale Police as a thin white male in his mid-30s, used the same type of knife in both incidents, police said. Police declined to give the names of the students who were robbed.

Perrotti said the robber has a beard or goatee and was wearing his hair in a ponytail. He wearing jeans and a dirty T-shirt at the time of the robbery, Sweeney said.

“The person told them that it was a robbery and took the money,” Perrotti said. “In both of these cases [the victim] cooperated.”

New Haven police were the first on the scene of the second robbery. Yale police later told city police about a similar case they had investigated earlier in the day, Perrotti said.

“At that time, we realized that it was quite possibly the same individual involved in both,” Perrotti said.

Perrotti said that New Haven police investigated the York Street robbery, but city authorities said they had no record of an incident at that location and time on Sunday.

New Haven police did report a third incident near campus at 10:30 p.m., when an assailant allegedly mugged a woman at 105 Church Street and stole her cash and ATM card, Sweeney said. She said the suspect in that robbery was also a white male with a knife.

On Monday afternoon, Perrotti sent an e-mail message to all Yale students informing them about the York Street and Science Hill incidents.

“We would ask you to be particularly alert to your surroundings, not to walk alone, and to report any suspicious activity or persons to the Yale Police,” Perrotti said in the message.

Dean of Student Affairs Betty Trachtenberg expressed concern that Yale’s campus was the site of armed robberies.

“I think it’s very horrible and very scary,” Trachtenberg said.

Perrotti said police have already made significant progress in the case but that it was too early to say what connection, if any, the suspect had to Yale.

Trachtenberg said she had complete faith that the police would catch the perpetrator.

“A really, really big-time investigation is going on,” Trachtenberg said.