Federal authorities said there's a "definite possibility" that two letters containing white powder sent to the Jacksonville offices of Florida's senators this week came from the same source.
The FBI said an analysis for both substances were negative for harmful substances, and the letters are undergoing forensic examination.
The Jacksonville Fire and Rescue Department hazardous materials team and Jacksonville Sheriff's Office bomb squad were called to Republican Sen. Marco Rubio's office on Tuesday afternoon after two staff members discovered a white powder in a letter.
The duPont Center on Prudential Drive was evacuated and two people were evaluated before the substance was deemed safe. Rubio was in Washington at the time.
On Monday, Democratic Sen. Bill Nelson's office on the 20th floor of the Riverplace Tower -- less than one-half mile from Tuesday's incident -- was evacuated after staff found a powder inside a threatening letter.
Nelson was in Jacksonville for a fundraiser but had already left his office by the time the substance was discovered. The powder turned out to be cornstarch.
There was no word on what the substance sent to Rubio's office was.
The FBI is investigating both incidents, telling Channel 4 that the bureau is "proceeding on the presumption they?re connected." The U.S. Capitol Police are also investigating.
An FBI spokesman said any evidence gathered will be presented for possible criminal charges.
Meanwhile, other elected leaders in the River City are beefing up their security measures as a result of the letters containing suspicious substances. The offices of Rep. Ander Crenshaw, R-Fla., and Rep. Corrine Brown, D-Fla., are taking extra precautions.
At Brown's office, in addition to having to be buzzed in at the front door, a security measure that took affect in January, workers are taking steps to thoroughly scan incoming mail.
"We'll stop him (the mailman) at the door, take the mail, and then use gloves, etc., at least for the next couple weeks," Ronnie Simmons, Brown's chief of staff, said in a phone interview.
U.S. Capitol Police provided the name and address from where the letters were sent to the senators to Brown's and Crenshaw's offices.
Simmons said that while Brown's office cannot provide that contact information as the investigation remains active, he said it's a good lead investigators are following and something to help guide the office staff as they sort through the day's mail.
"They also told us how to shake the mail and see if there was a powdery substance in it," Simmons said.
He said Brown's office is relieved the letters sent to both senators' offices turned out to contain harmless substances, but he said it could be some type of warning, and Brown's staff is taking it very seriously.