More white powder letters surfaced in North Texas and New England on Tuesday.
NBC 5 has learned that a letter was received at Boggess Elementary School in Plano on Tuesday morning and at schools in New York City, Rhode Island and Massachusetts.
Principal Marilyn Carruthers sent a letter to parents Tuesday alerting them to the letter and powder and of the police investigation.
In the letter, Carruthers said the school did not open the envelope because of similar packages that were received in the area and immediately followed safety procedures while storing the envelope in a sealed bag.
Carruthers added that tests by law enforcement revealed nothing harmful.
The letters received in New England also turned out to be harmless.
This comes after five letters containing harmless white powder were received in North Texas on Monday. The letters appeared similar to several other scares across the country on Monday and also could be linked to hundreds of unsolved hoaxes since 2008.
Powdery substances found on mail has been a concern since a series of anthrax-laced letters were mailed around the United States in the wake of the Sept. 11, 2001, terrorist attacks. Since that time, a number of hoaxes have been perpetrated.
Monday, the FBI confirmed agents are investigating suspicious white powder sent to Sunset High School in Dallas, Whitt Elementary in Sacshe, Wylie High School in Wylie, Gulledge Elementary in Plano and Texas Instruments in Dallas.
Also on Monday, white powder discovered in letters caused scares at a gallery in New York City, a bank building in Birmingham, Ala., a restaurant in Washington, D.C., and a middle school in Connecticut.
The U.S. Postal Inspection Service said several safety measures are in place and customer should feel comfortable that their mail is secure.
All of the mail that goes through North Texas' processing facilities go through a biohazard detection machine that scans the envelope for dangerous substances.
NBC 5 learned Monday that a letter sent to a gallery in Manhattan was postmarked in Grapevine.
"If a letter is actually postmarked Grapevine, that would mean it would have to come across the counter," said Amanda McMurrey, of USPIS.
There are cameras in post offices, McMurrey wouldn't elaborate on whether or not there may be surveillance video of the person.
The FBI would not discuss if the cases are related. Authorities said one of the letters sent to the Boston area on Tuesday came from Dallas.
NBC 5's Scott Gordon and Ray Villeda contributed to this report.