BOSTON (AP) — 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.

Police and emergency personnel respond the scene of an explosion in Boston, Tuesday, Sept. 13, 2022. A package exploded on the campus of Northeastern University and the college said a staff member suffered minor injuries. (WCVB via AP)

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.”

Northeastern in a message posted on its website Wednesday said campus is safe.

“Events such as the incident that took place on our Boston campus last night can create or heighten anxiety for many of us,” said the post, credited to Provost David Madigan and Chancellor Kenneth Henderson. “We would like to underscore what was communicated to our community last night: Multiple law enforcement agencies have determined that the campus is safe and secure.”

The campus opened normally for classes and other activities Wednesday.

Counseling and other support services were made available for students, faculty and staff.

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.

FILE – Students walk on the Northeastern University campus in Boston on Jan. 31, 2019. A police bomb squad sealed off part of the campus of Northeastern University late Tuesday, Sept. 13, 2022, to examine a pair of suspicious packages, and there were unconfirmed reports of an explosion and minor injuries to at least one person. (AP Photo/Rodrique Ngowi, File)

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.

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.