They say Joseph Quirindongo, 20, cornered his victims in elevators using a knife to force them into stairwells.
Police released surveillance video from an attack in April.
Investigators believe Quirindongo was also behind a sexual assault in a neighboring building, in January of 2010.
He's charged with attempted rape, assault and other charges.
The first attack happened on January 18, 2010 at approximately 7:30 a.m.
The 19-year-old victim was followed onto an elevator of an apartment building.
Once on the elevator, police say the suspect, armed with a knife, forced the victim out of the elevator and into the stairwell where he sexually assaulted her.
The suspect then fled down the stairs.
The victim was taken to a local hospital where she was treated and released.
Then on Thursday, April 14, 2011 at approximately 10:00 p.m. a 17-year-old victim was followed onto an elevator of an apartment building.
Once on the elevator, police say the suspect, armed with a knife, forced the victim out of the elevator and into a stairwell where he attempted to sexually assault her.
The victim fought off the suspect who then fled the scene.
