An FBI spokesman, Dave Fitz, told the Associated Press that agents had gone to the upscale townhouse. He did not immediately release any other specifics, citing an ongoing investigation.
The AP report that the agents could be seen inside the home Sunday evening near Baltimore's Inner Harbor, wearing vests and carrying guns.
Jacksonville Sheriff Mike Williams said earlier on Sunday that investigators believe Katz, who is from Baltimore, was the shooter who killed two people and then himself at a Madden NFL 19 tournament hosted inside a Jacksonville shopping center.
Previously, Baltimore Police Department spokesman T.J. Smith told the AP that he could only confirm that the police force was assisting law enforcement partners "with some information that has led authorities to Baltimore."
Katz, who is listed by EA Sports as a 2017 Madden tournament winner, was identified Sunday shortly after the shooting. He is believed to be staying in the area for the night as part of the tournament. While the FBI and ATF assisted the sheriff's office with getting more information about the suspect, local authorities impounded Katz's vehicle pending a search.
In addition to the two killed, nine other victims were transported to area hospitals for treatment, seven of whom were suffering from gunshot wounds. An additional two victims drove themselves to the hospital.
"I'm happy to report that they are all in stable condition at this time," Jacksonville Sheriff Mike Williams said at the press conference, noting that the number may change as they learn more from the hospital.
A Chicago man said that people trampled each other while fleeing the gunfire. Marquis Williams, 20, described people running in the panic to escape, according to the AP.
"Initially we thought it was a balloon popping, but there weren't any balloons in the room. Then we heard repeat shots and we took off running," Marquis Williams said.
Katz is believed to have used a single handgun in the attack. The motive is still under investigation, Sheriff Mike Williams said.0comments
"Tough day for us as you could imagine, but again we could not have been able to get to this point investigatively because of our partnerships [with other law enforcement]," he said.
Confirmation of the victim's identity has not been completed. The names of the victims who died will be released after their families have been notified, Williams says.