Dallas Fire-Rescue said tests determined that the the white powder in the letters was flour.
A letter with white powder that created a scare Friday at Love Field contained the same cryptic message as other letters sent since 2008 to two North Texas schools, dozens of governors across the country, and U.S. embassies around the world.
The FBI asked for public help when news of the mailings was first reported but has remained silent about the investigation as more letters have surfaced.
The mailings apparently started in December 2008, when someone sent letters containing white powder to at least 40 governors and 19 U.S. embassies from Spain to South Korea.
All the letters were postmarked "Dallas" or "North Texas," which indicates it was processed at the post office in Coppell, the FBI said at the time.
In a twist that made it appear the sender was taunting the FBI, the return addresses were current or former FBI offices, agents said.
In every case, the white powder turned out to be harmless. But the scares caused evacuations, the mobilization of hazardous materials teams, and in some cases, sent people to the hospital as a precaution.
In 2008, the U.S. Postal Inspection Service offered $100,000 for information leading to the sender's conviction.
On Friday, FBI spokesman Mark White said he could not comment on the investigation or any links between the more recent letters.
But they appeared strikingly similar.
In November 2009, white powder letters with the same references to the FBI and Al Qaeda were sent to at least seven foreign missions at the United Nations in New York. Employees who came in contact with the letters had to be decontaminated.
"Anybody who thinks it's a joke is making a very bad mistake," said an angry New York Mayor Michael Bloomberg.
On April 6, elementary schools in Garland and Sachse received white powder letters with the same one-line message, a school employee said. The schools were evacuated, and parents rushed to check on their children.
On Friday, the letter that surfaced at an American Airlines office at Love Field appeared to be nearly identical, law enforcement officials said.
A Dallas Fire-Rescue hazardous materials team found the white powder was harmless. Later tests determined it was flour, said DFR spokesman Jason Evans.
Sending threatening letters, even it it's a hoax, is punishable by up to 10 years in prison for each letter sent.