An early morning bomb threat at Tufts University in Massachusetts made reference to a labor dispute involving the school's janitors, a top university administrator said.

The handwritten threat found taped to the door of campus health services Monday morning remains under investigation and it's too early to draw any conclusions about who is responsible, Senior Vice President Mary Jeka said.

"I am terribly concerned for the safety of my community," she said, noting that local, state and federal law enforcement agencies are involved in the investigation.

The note was found shortly after firefighters responded to a car fire near health services at about 4:30 a.m. It was not clear if the fire and the note were connected.

The private university in Medford and Somerville closed several buildings referenced in the note, told students to stay in their dorms, postponed final exams and sent all non-essential staff home.

Tufts said in a statement that the buildings were evacuated and "multiple law enforcement agencies are actively working to clear" them. Bomb-sniffing dogs were involved, Jeka said.

A group of students known as the Tufts Labor Coalition has protested the university's decision to lay off 35 janitors who work for a private contractor. The university said the move was made to save money.

The group in a statement said it "had no role in, and condemns, these acts of violence."

The union that represents the janitors also condemned the threat.