U.S. Capitol Police said Tuesday they were investigating a letter with “concerning language” that was received near the office of Rep. Bennie Thompson, D-Miss., the chair of the House Jan. 6 committee.

Authorities initially responded to reports of a suspicious powder or substance, two law enforcement officials with knowledge of the matter said. Capitol Police later said in a statement that they did not find “anything dangerous inside.”

“Our investigators take all concerning statements and threats seriously,” the Capitol Police said on Twitter. “They are now working to determine who sent the letter and why.”

NBC News has asked Thompson’s office for comment.

The House committee investigating the Jan. 6 attack on the Capitol is scheduled to hold its next hearing Thursday. The panel postponed the public hearing late last month, citing Hurricane Ian’s pending landfall on Florida’s Gulf Coast. The hearing is less than a month before the midterm elections.

Also on Tuesday, Secret Service agents made a traffic stop near the White House and “came in contact with a powdery substance believed to be suspected narcotics,” agency spokesman Anthony Guglielmi said on Twitter.

One person was taken into custody and officers were being evaluated, he said. It is unclear whether the two incidents, at the Capitol and near the White House, were related.

Asked for additional details, Guglielmi said two officers were taken to the hospital after they were exposed to a horse tranquilizer. They had detained a person with an extradition warrant when they came into contact with the white powder, he said.