Earlier this morning, MIT's Emergency Alert system put out a notice from the Cambridge Police of a call that someone with a long rifle and body armor was in the Main Group Building on campus. The notice warned students to "shelter in place".  AP released this report:

BOSTON (AP) — The Massachusetts Institute of Technology says a man carrying a long rifle and wearing body armor was spotted in a campus building, and the school is on lockdown.

MIT said in a statement Saturday that several law enforcement agencies have responded, and a photo that appears to have been tweeted from near campus shows a road blocked to traffic.

The school advises students and workers to stay indoors and report suspicious activity to campus police.

More details were not immediately available.

Police tape surrounded the scene as officials searched for the gunman. Twitter lit up with news about the #MITgunman:

@Armyoftoys: Thanks to @twitter I found out there was a gunman on MIT campus before the police even got there I was IN the building the report came from!

@emilyyfordd: What the heck gunman at MIT

@girmayohannes: BREAKING! #MIT on lockdown after reports of armed gunman.

@anag_g: MIT on lockdown because of a gunman? That's my worst nightmare: Going to school and being shot. #GunControl NOW.

Police did not find a shooter and soon determined there was no immediate threat. They pulled down the tape, calling off the lock-down. Another report came from AP at 10:30 a.m.

BOSTON (AP) - Police in Massachusetts say a call reporting a gunman on the Massachusetts Institute of Technology campus was unfounded and that there is no threat to public safety.

Police said Saturday that officers searched for a man reported to be carrying a long rifle and wearing body armor and found nothing. A spokeswoman for the university says the school also called off a campus-wide lockdown.

WGBH will bring you updates on what happened on campus as we get more information.