Updated 03/06/13 – 11:03 a.m.

DEKALB, Ill. (CBS) – FBI agents, state police, and other law enforcement officials were at the Northern Illinois University campus on Wednesday to execute a search warrant.

CBS 2′s Marissa Bailey reports authorities raided the NIU police station shortly after 8 a.m.

A source confirmed to WBBM Newsradio’s Bernie Tafoya the investigation was linked to a rape case against a former NIU police officer accused of assaulting a student, and the firing of NIU Police Chief Donald Grady for misconduct stemming from his department’s handling of evidence in that case.

The FBI would only say the search warrant was connected to a criminal investigation being conducted by the FBI, Illinois State Police, the U.S. Department of Education, and the U.S. Department of Housing and Urban Development. She said the probe was not related to any public safety concerns.

Prosecutors dropped sexual assault charges against NIU Police Officer Andrew Rifkin last November, after accusing an officer of withholding statements that could have helped Rifkin. A grand jury re-indicted Rifkin last month, on allegations he raped an NIU freshman in October 2011.

The officer who failed to turn over the statements, Lt. Kartik Ramakrishnan, was placed on paid leave.

“For the FBI to be here, it has to be something huge,” Josanne Francis said.

“It’s not like they’re by someone’s house. They’re by university police and the public safety building. So how safe am I here that the FBI is at the building of the people who are supposed to be protecting me on campus?”