Students returned to two Montgomery County schools that had been evacuated on Tuesday morning after threats of gunfire and explosions, authorities said. Police said they searched the buildings and found nothing dangerous.

About 9:29 a.m., someone called the county’s 911 center and made threats of gun violence and bombs at Northwood High School in Silver Spring, said Angela Cruz, a Montgomery County police spokeswoman. Around that same time, a caller telephoned police and made similar threats for Northwest High School in Germantown.

Police and fire officials were investigating the source of the calls, a spokeswoman said.

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