The identity of the person sending white powder letters to hundreds of locations across the country remains a mystery Thursday.
The United States Postal Inspector said a letter recovered from a PostNet store Thursday morning does not match the style of previous white powder letters -- and it didn't contain any powder.
Investigators don't believe the man was trying to be a copycat, rather he was simply airing his grievances with local officials. The letters were determined not to be threatening and the sender will face no charges, officials said.
Thursday's investigation began after an employee of the PostNet store on the 18000-block of Preston Road alerted officials to a suspicious customer who asked for help with an angry letters to the FBI and Plano mayor.
The most recent letter was still at the facility and was turned over to authorities Thursday morning.
A Plano HAZMAT team was dispatched to the store and was later cleared.
Ashly Mason, the employee at the PostNet location, said a regular customer who frequently visited for copies came in twice recently wearing rubber gloves . On a visit last week, the man asked her to correct his spelling on the words "arrogant" and "ignorant" in a letter to the FBI that he asked her to type for him.
On Wednesday, he left another letter, which was addressed to the mayor of Plano, to be mailed the next day.
Mason said that night she saw the FBI's description of the man they believe is responsible for mailing out nearly 400 white powder letters over the last four years and that she felt the description matched her customer perfectly.
Thursday morning, Mason put the man's letter to the mayor aside and called the authorities.
Officials said that while that man wasn't the man they were looking for, Mason and PostNet did the right thing in sharing their suspicions.
On Wednesday, the FBI and U.S. Postal Inspector issued a $150,000 reward for information leading to the capture and conviction of the sender, whom FBI experts believe is from North Texas, at least 30 years old, may have a history of mental challenges and likely does not have a mastery of the English language. There have been no fingerprints on the letters, leading investigators to believe the man uses gloves when preparing and mailing the letters.
Letters containing white powder invoke fear in the recipients after five people were killed and more than a dozen were injured when letters laced with anthrax spores were mailed to offices in the days and weeks after Sept. 11, 2001.
Since then, hundreds of hoax letters have been received nationwide at schools, banks and other businesses. Though they've been harmless, anyone receiving a white powder letter is advised to evacuate the area and call 911.
Earlier Thursday, a white powder letter was reported at about 8:30 a.m. on the 6th floor of the Frank Crowley Courts Building in Dallas. The building was not evacuated and no injuries were reported, Dallas Fire-Rescue said. Tests on the powder determined it posed no threat.
NBC 5's Kendra Lyn and Ray Villeda contributed to this report.