A hard plastic case that exploded on the campus of Northeastern University in Boston causing minor injuries to a staff member contained a rambling note that railed against virtual reality and also referenced Facebook founder Mark Zuckerberg, a law enforcement official said Wednesday.

Investigators are still trying to identify the motive behind the Tuesday evening explosion and are working to understand why the package was specifically sent to Northeastern, the official said.

Investigators are also trying to determine the specific mechanism of the device and whether the package contained any gunpowder, the official added. The official could not discuss details of the investigation publicly and spoke to The Associated Press on condition of anonymity.

The official described the case as a “Pelican-style” case. Pelican makes hard cases designed to protect sensitive equipment.

A spokesperson for the FBI office in Boston declined to comment Wednesday, saying the investigation is “still very active and fluid.”

A Northeastern spokesperson on Wednesday had no new details on the case.

The package delivered to Holmes Hall detonated just after 7 p.m. Tuesday when a staff member opened it, the university said in a statement. The staff member, a 45-year-old man, was taken to the hospital with minor injuries to his hand, police said. No name was made public.

Boston’s bomb squad neutralized a second package near the city’s Museum of Fine Arts, which is near Northeastern’s campus.

Holmes Hall is home to the university’s creative writing program and its women’s, gender and sexuality studies program.

Northeastern is a private university in downtown Boston with about 16,000 undergraduate students. The campus opened normally for classes Wednesday.

Tuesday’s explosion marked one of the first big scares in Boston since 2013, when two bombs planted near the finish line of the Boston Marathon killed three spectators and wounded more than 260 others.

(AP)