The University of California, Riverside, canceled final exams and a graduation ceremony Friday evening after finding a homemade firebomb on campus and receiving two threatening letters.

"The campus is taking the threat seriously," school officials said in a statement released late Friday afternoon.

School officials decided to take action after groundskeepers found two plastic bottles that possibly contained flammable liquid near a burning palm tree outside the Life Sciences building around 5:25 a.m., said campus Police Chief Mike Lane.

Another suspicious device was found hidden in a planter near that building three hours later, and two handwritten, threatening letters were received in the afternoon, Lane said.

The second letter "reiterated a threat against tonight's commencement," he said.

Five other graduation ceremonies planned for the weekend would go on as scheduled under increased security, and Friday's commencement activities for the College of Humanities, Arts and Sciences would be rescheduled, said university spokeswoman Kim Lane.

The university has several schools that hold their own individual ceremonies.

"The campus is working with law enforcement agencies, including UCR police and the Riverside Police Department to ensure that the commencement ceremonies remain a safe environment for those attending," the university said in a statement posted on its Web site