Island Avenue School in Madison was in lockdown on Wednesday after a powdery substance was found in the mail.
The substance was found in the morning and students and staff were removed from the area and people who were in contact with the substance were isolated, according to police.
Students and staff at the elementary school were safe and no one was hurt, school officials said, and they alerted parents through an email.
The envelope was removed from the school and tests have not shown anything so far, according to police.
Additional tests are being conducted, but the results are not expected for several weeks.
The lockdown was lifted as of 1:15 p.m. and Madison police, the FBI, U.S. Postal Inspectors and the state police are investigating.
This is one of three schools in the state to receive a letter with suspicious powder this week.
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Anna Reynolds School in Newington was evacuated Wednesday after a letter with powder turned up there.
Kennedy Middle School in Enfield was locked down on Monday after school staff found a suspicious envelope in the main office that had white powder.
Police were testing the substance and school officials said the results are expected to take about a week.
Enfield Supt. John Gallacher posted a notice online saying the FBI is investigating after schools in several states received letters containing white powder that were sent through the U.S. mail.
