The mayor said a call came into a New Haven non-emergency number just after 1:30 p.m. saying bombs were left at seven buildings on campus.
Out of an abundance of caution, several buildings were evacuated until all areas were deemed safe.
New Haven Police said the area would be shut down for hours during their investigation, but they gave the all clear around 7 p.m.
The Department of Homeland Security assisted local and campus police.
Yale Police Chief Ronnell Higgins said the person who made the bomb threats did not give a reason why.
Yale spokesperson Karen Peart said the campus buildings that were threatened included Bingham Hall, Branford College, Grace Hopper College, Jonathan Edwards College, University Theatre, Vanderbilt Hall, Welch Hall and Yale Art Gallery. Bass Library and Saybrook College also was evacuated as a precaution.
City officials urged people to avoid the downtown area. They said they were hoping to clear the buildings and reopen streets by early Friday evening, but said the process could take much longer.
(The Associated Press contributed to this report.)
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