DEMAREST, N.J. (CBS 2/WCBS 880) — Students began school Tuesday under a heavy blanket of security at North Valley Demarest High School.

Dozens of police stood guard and led thousands of students through a metal detector one at a time. The heightened alert comes after police got a tip that someone was threatening to bring a gun to school and open fire.

READ MORE: Nor'easter Triggers Blizzard Warning In Suffolk County, As New York Braces For Widespread Snowfall

Investigators were still trying to figure out who was behind the threat.

WCBS 880’s Sean Adams reports

“I was scared.  Very scared.  I didn’t expect it at all,” said junior Julie Singer.

“A group of individuals were overheard that they were going to shoot up the school today,” said Demarest Mayor James Carroll. He said questions answered at the school’s tip line made the threat more credible than most. Police were alerted sometime on Monday.

READ MORE: New York City Braces For Major Winter Storm; DSNY Pretreats Roads, Has 1,800 Snow Plows Ready

Demarest and Northern Valley Regional District’s other high school, Old Tappan, were both swept overnight. Canine units were brought in to sniff out explosives and weapons but, CBS 2HD was told, they did not find any.

Police issued a reverse 9-1-1 call Monday night alerting parents. Shortly before midnight, school administrators posted a letter on their website saying: “The district received a We Tip warning that a potential school shooting may occur sometime during the week of November 8 through 12. Based upon the advice of law enforcement students will not be permitted to leave the building for lunch, free periods or physical education or for any reason.”

Students and parents were on edge Tuesday morning. “You think about what’s happened over the past years, for sure.”

Asked what she was thinking about, junior Haleigh Hammer answered: “what happens if something really does happen if we’re at school and how they’re going to react to that?”

MORE NEWS: State Of Emergency Declared In New Jersey Ahead Of Nor'easter