UPDATE: The Globe reports  it was a hoax "electronic message." WBZ reports  the FBI and the Secret Service have joined the investigation, while campus police try to explain why it took them more than an hour to issue an alert.
At 8:47 a.m., MIT Police issued an alert:
This morning information was received from Cambridge Police that there was a person with a long rifle and body armor in the Main Group Building of MIT. Multiple law enforcement agencies have responded, stay indoors and shelter in place and report suspicious activity to the campus police dispatch dial 100.
Mass. Ave. was shut near the building and Route 1 buses were diverted.
At 9:33, Cambridge Police tweeted :
Police responded to report of man w/ gun inside building on Mass Ave. Police searched building w/ negative results.
By 9:50, police were taking off their bullet-proof vests .
At 10:19, Cambridge Police tweeted :
Scene is clear. Call unfounded. No threat to public safety.
- Cambridge 
- MIT