DALLAS (CBSDFW.COM) – A lockdown has been lifted after a Dallas high school got a scary piece of mail.

According to DISD spokesperson Jon Dahlander, a threatening letter was opened around 10:30 Monday morning at Sunset High School.

In addition to a letter, the envelope contained a suspicious white powder.

A hazardous materials team was dispatched to the school and discovered the powder was cornstarch.

“You can never be too sure in these situations –- nine times out of 10 they’re hoaxes,” said Dahlander, “but on that tenth time you just need to be careful and make sure that you took all the right steps and we did today.”

During the incident, students and personnel in the general area of where the envelope was opened were moved, but the entire school was not evacuated.

Ad for details of the letter Dahlander said, “I’m not gonna get into the contents too much, but it was a threat to an individual that they just ‘better watch’ what they’re doing.”

The U.S. Postal Service is investigating.

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