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

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

They are also trying to determine the specific mechanism of the device and whether the package contained any gunpowder, the official added.

The official described the case as a “Pelican-style” case.

Pelican makes hard cases designed to protect sensitive equipment.

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

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

The package delivered to Holmes Hall detonated just after 7 pm on Tuesday when a staff member opened it, the university said in a statement.

The worker, a 45-year-old man, was taken to the hospital with minor injuries to his hand, police said.

Boston’s bomb squad neutralised 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 programme and its women’s, gender and sexuality studies programme.

Northeastern is a private university in Boston city centre with about 16,000 undergraduate students.

The campus opened normally for classes on 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.