Adam Lanza, the gunman who killed 20 children and six adults in an elementary school shooting in Connecticut, acted on his own, a year-long inquiry into the massacre has ruled.
The report, published today, admitted that questions about Lanza's motive may "never be answered conclusively."
Police are going door to door at Yale University to search for a gunman on campus, a spokesman has warned.
Yale University urged students and staff to co-operate, warning, "When they knock on your door, a Yale police officer will slip their Yale ID under the door".
"Police may use keys, but they will identify themselves", the university website added.
Libraries and some public schools are also on lockdown as the search continues, New Haven Police Department spokesman David Hartman said.
New Haven Police Department received an anonymous tip off from a male caller that his room-mate was en route to the Yale University campus "with the intention of shooting people", a spokesman said.
Police spokesman David Hartman told reporters both the caller and the gunman have yet to be identified.
He told a press conference: "There has been no report of gunfire, there has been no report of injuries of anyone.
"We are interviewing [two] witnesses who did report seeing somebody on or near the university campus with a long gun ... either a rifle or a shotgun".
Officers from the Yale Police Department, New Haven Police and Connecticut State Police are "actively searching for any gunman" on the scene at the university's Old Campus, Yale's website states.
Issuing a renewed alert, Yale University warns its students and staff that the lockdown continues, adding, "THIS IS NOT A TEST".
Anyone with information on the gunman have been urged to call the emergency services.
Yale spokesman Thomas Conroy told local newspaper the New Haven Register, “Everyone is following instructions they were given in these alerts".
The Yale alerts warning an emergency situation is in place are sent by email, text message and phone.
Everyone in our community receives these alerts ... Everyone is alerted in multiple ways,” he added.
Students at Yale University have been on break since Saturday and classes do not resume until 2nd December.
Yale University has warned its students and staff it remains on lockdown and they should continue to "shelter in place" if on campus.
"IF OFF CAMPUS, STAY AWAY", the university's website warns.
New Haven Police Department Lieutenant Jeff Hoffman confirmed to student newspaper Yale Daily News there has been no shooting or any injuries as police search for a gunman on Old Campus.
Yale Police Department and New Haven Police have blockaded a road near to the campus.
SWAT teams are reportedly assembling outside Yale University's Old Campus following the confirmed report of a gunman on campus.
Yale Daily News, the student newspaper, also reported that announcements have been broadcast in dorm rooms warning of the situation:
SWAT teams sighted on Old Campus.
Another in-room PA announcement was just blasted across dorms out reporting gunman on Old Campus.
Yale University in New Haven, Connecticut, said local police received an anonymous call from a telephone box this morning reporting there was a person with a gun on the campus.
Yale and New Haven Police are currently in the area, the university added.
Yale University in New Haven, Connecticut, has urged students and staff to take shelter after local police received a report of a person on campus with a gun.
"Confirmed report of a person with a gun on/near Old Campus. SHELTER IN PLACE. This is NOT a test", the university website reads.