BOSTON (WHDH) - After a detonation at Northeastern University injured a campus staff member, law enforcement sources are revealing more information about the contents of that package– but still don’t know who sent it.

Federal agents and investigators are now scrambling to figure out who left the package inside Holmes Hall on campus that detonated at around 7 p.m. Tuesday night, injuring a 45-year-old staff member’s hand and sending him to the hospital.

The crime scene was sealed off for hours and students were evacuated from nearby buildings. The school also ordered students to shelter in place until the bomb squad completed its sweep. They declared a second package found in the area safe.

Law enforcement sources said that the package didn’t contain any explosives, but had been pressurized to detonate when opened. Sources also confirmed that a crumpled one-page letter dated from Tuesday was addressed to the lab manager. It railed against virtual reality and Facebook founder Mark Zuckerberg. The letter mentioned two other locations, but bomb technicians found no other threats.

The packages are presented in an enticing way to get the victims to open them, and only detonate when they’re opened.

Police are looking at campus surveillance footage to determine when the packages were left on campus, and are working with agencies including the Joint Terrorism Task Force and the FBI’s Evidence Response Team to crack the case.

The campus is now secure and students have returned to class. The school added that counselors will be available for students who have been affected by this incident.

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