Two young men, one armed with a knife, raped a University of Minnesota student in her dorm room late at night over the weekend, authorities said Tuesday.

No arrests have been made, and students who live in that dorm and others are questioning the safety of buildings and the campus as a whole.

While this is the sixth reported sexual assault — including at least three rapes — on or near campus since mid-February, it is unusual because of its location. In the 15 years he has worked at the university, said U spokesman Tim Busse, this is the first armed sexual assault of a student in a dorm that he can recall.

Sexual assaults by strangers are “very rare” in dorm settings, said Katie Eichele, director of the Aurora Center for Advocacy and Education, a crisis intervention group for victims of sexual assault at the U.

When assaults occur within residence halls, it’s almost always by an acquaintance, she said.

The assault occurred between 12:45 and 1:30 a.m. Sunday in Sanford Hall, in the 1100 block of University Avenue SE., according to campus police.

The victim, a freshman, reported the assault to U police the next day and said she did not know her attackers.

Busse said campus officials are trying to learn how the men got into the building, which has electronic and security personnel measures in place, before they reached the student’s room on the fourth floor.

He acknowledged that students routinely “prop doors open and let others tailgate in” through exterior doors, making no dorm building absolutely secure.

He also didn’t rule out that one or more of the men could be students.

Police are reviewing campus video that might help them determine the identity of the suspects, Busse added.

“Unfortunately, we have very limited suspect descriptions at this time,” Busse said, saying only that the student described her attackers as 18 to 21 years old.

The university said in late February that it was raising the bar on the specificity needed before suspect descriptions would be released to the public.Jenae Herron, 19, also a freshman in Sanford Hall, said, “It’s just kind of frustrating to know that she was just walking in the hallway. … In the place that I’m living, I have to be on guard.”

Herron said it’s easy for people to sneak into dorms because anyone can just follow someone else entering. Herron said she generally feels safe on campus.

She takes advantage of the security monitor program that allows students, staff and visitors to request free security escorts to and from campus locations and nearby neighborhoods.

Lamin Manjang, a 21-year-old junior waiting for the bus outside Sanford, said, “It’s kind of sad when we live in a world where a female can’t walk home by herself.”

A rash of sexual assaults and violent robberies starting in fall 2013 and continuing into February 2014 set the campus on edge, prompting university officials to heighten security, adding lights, police patrols and new security cameras. Most of the crimes, however, targeted students walking off campus at night.

Dara Salour and Vishnu Namboodiripad, both 19 and freshmen who live in Sanford, said they were surprised to hear about the attack.

“It was behind two sets of doors. How much safer can it get? … I don’t know what else [the U] can do,” Salour said.

Logan Eickhoff, also a freshman in Sanford, said she feels safe in the co-ed dorm, which requires students to scan their IDs to enter the lobby after 8 p.m. and again to access living quarters.

The attack won’t change her routine, Eickhoff said, but reinforces the need for being “extremely cautious about making sure her door is locked.”

According to Tuesday’s campus alert:

The student was in the hall outside her room “when she encountered two males who walked her back to her room.”

Once in the room, one of the suspects threatened her with a knife and both of them sexually assaulted her.

Busse said the woman underwent a sexual assault exam at a hospital.

Anyone with information about the assault is asked to call the campus police at 612-624-2677.

No arrests have been announced in any of the other recent incidents. However, there is no indication that this weekend’s attack is related to the earlier assaults, Busse said.

Staff writer Liz Sawyer contributed to this report.