CHICAGO (CBS) — A threat prompted authorities to evacuate the busy Clark/Lake CTA ‘L’ station late Wednesday afternoon.

Loop trains were halted for about half an hour before an all-clear was given around 4:45 p.m.

Police said they were investigating the viability of a threat to the Clark/Lake station. Commuters were advised that there would be a heavier-than-normal police presence.

The threat proved to be unfounded, police said.

The Clark/Lake station is composed of an elevated platform alongside the Thompson Center that serves the Pink, Green, Brown, Orange, and Purple line stations, and a Blue Line subway platform.

The CTA said trains on all those lines were left standing near Clark/Lake after the threat was received.

Blue Line service was shut down, and O’Hare-bound Blue Line riders were told to evacuate at Washington. No reason was provided to the riders.

CTA Evacuation

Police activity prompted the evacuation of Loop L and Blue Line subway stations on Wednesday, Sept. 25, 2019. (Credit: CBS 2)

Some CTA passengers expressed confusion on Twitter – reporting that there were no trains going into or out of the station and that they were not told why the station was evacuated.

@cta what’s happening at Clark/Lake? Everyone was removed from the train and not allowed to board.

— Eloisa (@MissGeeklyChic) September 25, 2019

Thompson Center workers were also told toe evacuate, CBS 2 Director of Digital Content John Dodge reported. They said they were not told why.

CTA Evacuation

Police activity prompted the evacuation of Loop L and Blue Line subway stations on Wednesday, Sept. 25, 2019. (Credit: CBS 2)

Following the all-clear, residual delays were expected. But CTA service was resuming on all lines impacted.