It's being called a hoax, but FBI officials say they've developed leads on who may have started it.

What turned out to be quite a scare in downtown Bowling Green Monday morning quickly spread to other parts of the state

The FBI now says it's likely letters sent to four congressional offices in Kentucky said to contain anthrax may have come from the same source.

The offices of U.S. Representatives Geoff Davis, John Yarmuth, Ed Whitfield, of Hopkinsville, and Brett Guthrie received the letters.

Investigators have determined the threat was a hoax, but it's not stopping authorities from prosecuting to the fullest.

"We're still going to seek prosecution on whoever sent this," said James Hendricks, resident agent for the FBI, Louisville Division.

Artificial sweetener--that's the substance emergency officials say caused concern at the Warren County Justice Center.

"They came in to check their mail from a long weekend and found the letter in the mail," Hendricks said.

Four workers inside Congressman Brett Guthrie's office were quarantined to the third floor after opening a threatening letter claiming it contained anthrax.

More than 100 people were evacuated.

"I've been standing here talking to people who had court dates, and came from out of town and they've been canceled because of this foolishness," Congressman Guthrie said.

Guthrie, who was in Bowling Green Monday morning, calls the incidents unfortunate for both the city and the state.

"It's just sad that people will do this. It's disruptive. The people in this office are the ones who help veterans, the ones who help people with social security," he said.

Guthrie says a Hazmat team was able to determine the threat a hoax using equipment Bowling Green was able to receive through homeland security funds, quickly easing concerns.

"And it was checked out within 15 minutes of going in. It took an hour to get in, but otherwise it would've had to been sent off and probably had the building sealed for a couple days," Guthrie said.

And as the investigation continues by the FBI, Congressman Guthrie says for him, it will be back to business as usual.

"Our office will be back open. We have people to serve."

The U.S. Attorney's Office says if convicted, a person would face federal charges for sending the letter.

James Hendricks says he's working with other agencies around the state in an effort to find that person.