A Saybrook College senior was lying in bed with the lights out at about 3 a.m. Thursday morning, trying to get some sleep after a night out, when an unexpected visitor arrived.

The door to the senior’s fourth-floor single bedroom opened, and she heard someone moving around her room. After about a minute, the visitor left, leaving the student and her room undisturbed. Thinking it must have been a friend, she thought nothing of it and went to sleep.

But the next morning, the senior read an e-mail from Saybrook Dean Paul McKinley and “freaked out.”

“We write to report,” the e-mail began, “that an unidentified intruder entered a number of suites in Saybrook last night.”

Yale Police spokesman Lt. Steven Woznyk said in an e-mail that police officers responded to Saybrook around 3:30 a.m. in response to a complaint about the suspicious person. Woznyk said the intruder, who is male, went into at least two suites and left after he realized that one of the occupants was awake. As of press time, police had not found a suspect. Nothing was taken and no one was harmed, Woznyk added.

In a second e-mail to Saybrook students just before 5 p.m., McKinley called the intruder’s entry “an isolated incident,” adding that the incident likely involved a Yale student with keycard access. Woznyk said Yale police are investigating whether the intruder was in Saybrook. Yale Police and University officials have long said that students not locking their doors is a main cause of incidents such as Thursday’s, sometimes with more serious consequences.

This past February, another intruder was able to enter Saybrook suites. Police arrested 23-year-old New Haven resident Kevin Smith, who was also arrested on campus in 2007 for carrying a stolen credit card and marijuana. In July, Smith was sentenced to 10 years in prison.

In an e-mail Thursday, Saybrook Master Paul Hudak declined to comment on whether the college would take any steps to prevent further intrusions, citing the ongoing police investigation.

Thursday’s incident followed the summer installation of locks on Jonathan Edwards College bedroom doors by the Office of Facilities, in a move to amp up security. The University approved campuswide installation of the locks in 2008, after a spate of petty thefts the previous year. Branford Master Steven Smith said Wednesday that the locks would fix the problem of theft in dorms.

“Students are extraordinarily careless about locking doors,” he said. “They believe they live in a completely safe and secure environment.”

Seven Saybrook students interviewed Thursday night had mixed reactions to locking their doors, though four said the intrusion would make them take additional security precautions.

“Our door isn’t locked, but we probably will now,” Nicholas Leingang ’13 said.

The entryway and suite doors leading to the dorm of one Saybrugian were broken until Thursday, when the student read McKinley’s e-mails and sent one to Master Hudak, the Saybrugian said. Hudak replied that immediately installing locks for the bedrooms of individual students would not be possible, according to the student.

“I would install them myself, but I’d probably get fined,” the student added.

Woznyk said the incident was a reminder to students to lock their doors and to report any suspicious activity immediately to Yale police.

As for the Saybrook senior whose suite was entered Thursday morning, she said she will probably stop propping open the door with a hanger.