Letters containing mysterious powder sent to Trump, Supreme Court Justice Thomas

WASHINGTON -- A series of threatening letters containing a mysterious white powder is raising concerns that Republican lawmakers in Kansas and across the nation who received them are being targeted by someone cryptically calling themselves "your secret despirer," according to authorities and recipients of the letters.

Since Friday, about 100 letters have been received by Kansas GOP lawmakers and Kansas Attorney General Kris Kobach, who is also a Republican, officials said.

The list of intended recipients of white powder letters is growing and expanding beyond Kansas, federal law enforcement sources told ABC News.

Similar letters containing a cryptic note and a "suspicious powdery substance" were addressed to former President Donald Trump, Supreme Court Justice Clarence Thomas and other high-profile individuals, though the U.S. Postal Inspection Service intercepted them before they were delivered, the sources said.

Postal inspectors have possession of the letters and have deemed them harmless, the sources said.

At this point, the FBI and the U.S. Secret Service are monitoring the situation.

"There is some message. The message is somewhat unclear, but it was intended to be threatening," Republican state Sen. Molly Baumgardner, the recipient of one of the letters, told ABC Kansas City, Missouri, affiliate KMBC.

The FBI and the Kansas Bureau of Investigation are probing the letters, but have not determined who is sending them or the motive behind the dispatches.

"Kansas legislators that are Republican are being targeted," Baumgardner said.

Baumgardner said the letter mailed to her contained a white powder and a typed note reading in part, "It is important not to choke on your ambition." The sender described the letter as a "gift" and referred to themselves as "your secret despirer," a word that stems from despise.

"Everybody has to be concerned," Baumgardner told KMBC. "Everyone has to take this and any subsequent threats like this very seriously."

Investigators have collected the letters and a small sample of the white powder contained in them has been sent to a lab for testing.

"Preliminary tests have returned from this lab indicating the substance is presumptively negative for common biological agents of concern," the Kansas Bureau of Investigation said in a statement over the weekend.

The substances tested have not been deemed explosive, a source familiar with the matter told ABC News.

"Currently, no injuries have been reported, but we ask everyone to remain vigilant in handling mail," the Kansas Bureau of Investigation said in its statement.

Kansas state Rep. Steve Owens, a Republican, told ABC News that he received a letter similar to the one sent to Baumgardner. He said that once he opened the letter, mailed in a standard white envelope, he noticed the white powder inside a folded piece of paper.

Owens described the experience of receiving the letter as "terrifying."

He said the sender appears to be thorough and calculating, using the return address of a local church to seemingly try to coax the receiver into opening it.

"This is not okay," Kansas state Rep. Tory Blew, a Republican, wrote in a social media post after she also received a letter with white powder inside. "I'm very thankful for our first responders -- words can't describe my gratitude after this event."

The letters came after Manhattan, New York, District Attorney Alvin Bragg, a Democrat, was sent at least two letters containing white powder in April after a grand jury indicted former President Donald Trump on 34-counts of falsifying business records related to a hush money payment made in the final days of the 2016 presidential race to bury negative publicity about alleged extramarital affairs. The New York Police Department determined that the powder in the envelopes sent to Bragg was nonhazardous.

Investigators have found no evidence linking the Kansas letters to the Trump case in New York. Trump pleaded not guilty to the charges.