Sandy Hook Elementary School in Newtown, Connecticut, was evacuated Friday morning after receiving a bomb threat on the sixth anniversary of the mass shooting that left 26 students and teachers dead.

The Newtown Police Department said it received a call around 9 a.m. ET that a bomb was at the school. Officers were sent to the school and conducted a search as a precaution, but do not believe the threat was credible.

Police Lt. Aaron Bahamonde said the school was evacuated as a precaution and the school's superintendent thought it best to send students and staff home for the day.

On Dec. 14, 2012, a former student, Adam Lanza, shot and killed 20 first-graders and six educators before killing himself as police closed in. He had shot and killed his mother, Nancy Lanza, before going to the school.

The threat Friday came a day after police departments across the country and in Canada were alerted to bomb threats sent to businesses, hospitals and other places. Authorities in both countries said the reported threats were either emailed or phoned in.

On Friday morning, the Detroit Police Department said in a series of tweets that it was investigating multiple threats targeting two courthouses and three hospitals.

"At this time it is believed that these threats were received by phone," the department said, later adding that its bomb squad had investigated four of the locations and did not find any explosive devices.

Police in other cities, including New York, Oklahoma City, San Francisco and Washington, were also investigating bomb threats. In a series of tweets, the New York Police Department said it was monitoring "multiple bomb threats that have been sent electronically to various locations throughout the city."

The department said none of the threats appeared credible.

In Oklahoma City, a police spokesman said 10 to 13 email bomb threats were sent to businesses and other places in and around the city. The department said investigators did not find anything serious.

Threats targeting a Jewish community center in San Francisco and multiple branches of the San Francisco Fire Credit Union led to evacuations Thursday. Authorities in Los Angeles said it had received at least two dozen threats.

Newtown police said they do not believe the threat at Sandy Hook is connected to the ones received nationwide.