Nassau County police stepped up patrols at all schools Monday and urged educators to “be alert” after receiving a bomb threat — but law enforcement emphasized the threat was “unconfirmed” and “probably not credible.”

Nassau Chief of Department Steven Skrynecki said Monday that an unidentified man called the Second Precinct about 8:30 p.m. Sunday and said he overheard another man saying “he had placed bombs in every school in Nassau County and was planning to detonate them today.”

The police department on Sunday night notified superintendents for all the county’s 56 public school districts, as well as officials from the more than 100 private schools and the dozen or so colleges to the threat, Skrynecki said, adding that the news media was also alerted.

“It is probably not credible and quite frankly probably unrealistic to expect that somebody could place bombs in every school in Nassau County. The police department still thought it was worthy of passing this information onto the public and the school districts,” Skrynecki said.

The chief added: “This was a situation where we clearly did not want to alarm parents, because we do not think this was a credible threat, but at the same time we’re walking that fine line of keeping people informed and not alarming them.”

Nassau police directed patrol officers to visit schools in their patrol area on Monday and also alerted the county’s village and city police departments and spoke to officials from the Suffolk County Police Department, Skrynecki said.

Administrators at Paul D. Schreiber High School in Port Washington advised the school community that they were working with local police and “should the need arise, we will contact all families with further information.”

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The Oyster Bay-East Norwich Central School district informed its school community that security had been stepped up at each building — that the interiors and perimeters had been checked and that “appropriate staff is reviewing video camera footage.”

Skrynecki said one school in Island Park, following police direction to be on alert, called cops when someone spotted a cellphone lying in a doorway. It was not a threat, he said.

Nassau detectives said anyone with information could call 911 or Crime Stoppers at 800-244-8477.