A handwritten note threatening a Wednesday shooting at Western Illinois University prompted increased police presence on campus, while students were given the option of skipping classes without penalty, officials said.

The threat left at an apartment near the 11,000-student campus triggered a university alert system and comes slightly more than a month after a gunman killed five students and himself in a crowded lecture hall at Northern Illinois University in DeKalb, just about 150 miles northeast of Macomb.

"That's the first thing that comes to your mind," said WIU spokeswoman Bonnie Barker. "You don't want that here."

An employee at an off-campus apartment complex received the note that was left in a drop-box sometime late Tuesday or early Wednesday, university officials said. WIU and Macomb police officials declined to provide details about the note, including its exact contents.

No suspect was in custody Wednesday afternoon, Macomb police said.

WIU officials told students they could choose not to attend Wednesday classes but stopped short of canceling classes or imposing a full lockdown, because they concluded "the threat was not pointed enough," Barker said.

WIU said residence halls were locked and only accessible by residents with keys.

"While this is an anonymous off-campus threat, it is imperative that we take all necessary precautions to ensure the safety of our students, staff and faculty," WIU's president, Al Goldfarb, said in a statement.

Illinois State Police, Macomb Police Department and the McDonough County Sheriff's Office were assisting WIU.

"We are investigating this incident, and we will prosecute the individual responsible to the fullest extent of the law," Goldfarb said.

The WIU activated an alert system at around 9:30 a.m. Wednesday, shortly after the note was found, Barker said. The system, introduced last year after a shooting at Virginia Tech University that left 32 people dead, includes automatic alerts by cell phone text message and e-mail.