It was a very alarming day for some North Side residents Wednesday who had no warning before dozens of federal agents descended onto their block, swarming a two-flat, for what now appears to be a terrorism related investigation.

The raid took place in the 5200 block of North Virginia Avenue in the Lincoln Square neighborhood.

Eyewitnesses tell NBC 5 the federal agents, some in flak jackets from the FBI and JTTF, raced up the stairs and about an hour later came down with one man who neighbors believe is a ride-share driver. Other neighbors had long been curious about the second floor apartment in particular.

“There used to be like paper that covered the windows," said neighbor Katie Wienke.

“It’s just scary because we never saw anyone inside or outside of the building," Susana Luna, another neighbor, said.

Another neighbor told NBC 5 for days if not weeks preceding this raid they had noticed someone watching the block from his car like a detective might do.

The Chicago Tribune quotes a source saying what happened here was a matter of national security and that a search warrant was being executed at the residence. Neighbors say it’s rental property with regular turnover.

After watching agents in and around this two flat for hours Wednesday, one neighbor approached an agent to ask what had actually happened.

He said the agent replied “I don’t know what you’re talking about” got in his unmarked car and drove off like the rest of them.