A letter left in Nassau County mailboxes threatening people with arrest if they're outside during mosquito spraying -- it's a hoax, authorities warned.

The letter, signed by "Supervisor Charles Mangano," states spraying will be done by helicopter for the next five Saturdays, starting this weekend, and that police will patrol the area to arrest anyone outside during that time, county officials said.

"On the day of your town being sprayed, it is important for all children to stay inside their houses with the doors and windows closed," according to the fake notice, which has several spelling and grammatical errors. "Please do not answer the door. Spray is extremely dangerous and could be deadly. All children must be inside there house from 9:00 am until 9:00 pm. Police will be driving in the area to arrest any person outside during that time."

StoryRead the full hoax letter

It ends with "remember any person outside at the time of their town's mosquito spraying will be arrest and put in jail for their own safety."

All that is "completely false," said Nassau health and police officials as well as County Executive Edward Mangano's office.

Several residents in Glen Cove and Sea Cliff areas called the health department Friday after opening envelopes containing a letter without any county seal or letterhead, said Mary Ellen Laurain, county health department spokeswoman.

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Those communities are among the ones on Saturday's spray list in the fake notice.

Spraying is not needed because mosquito season is practically over, she said.

"There would be no public health reason to do that," Laurain said.

When spraying is scheduled, the county notifies the public through municipalities and other ways, including automated phone calls, news alerts, social media, website postings and the Nassau Now app.