Three Colleyville schools and Lamar High School in Arlington were locked down Tuesday in separate incidents while police searched for weapons.
None was found at any school.
In Colleyville, an anonymous note made a possible threat to safety at Colleyville Heritage High School, officials said.
Lockdowns were ordered out of "an abundance of caution" at the high school and nearby Heritage Middle School and Heritage Elementary School.
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But the lockdowns ended after about 50 minutes, and students were released at normal times, officials said.
"The Colleyville police assisted us with release, and everything went great," said Joe Harrington, principal at the high school. Evening activities were to continue as scheduled and, "school will be on a normal schedule tomorrow."
In emails to parents, Harrington said a student found an anonymous note in the cafeteria during lunch and immediately took it to the front office.
"It contained a very vague threat against Colleyville Heritage," Harrington wrote. "We immediately contacted Colleyville police."
They called for the lockdown around 3 p.m. "as a safety precaution."
School Superintendent Robin Ryan said in a letter to parents that the note "referenced the possibility of weapons."
Therefore, he wrote, "The district also has called in a third-party service to sweep the building using a dog trained to detect gunpowder-based weapons."
No weapons were reported from the sweeps.
"Additionally," Ryan wrote, "we will have an increased presence of police and administration on these campuses throughout the week."
Earlier Tuesday, a lockdown at Lamar High School in Arlington was ordered for more than three hours after a student told a school resource officer that a classmate might have brought a handgun to school. Police searched, but no gun was found.
Staff writer Nicholas Sakelaris contributed to this report.
Bill Miller, 817-390-7684