Police arrested a man late Wednesday night whose actions they believe triggered the evacuation and full-scale search of Calgary's tallest building one day earlier.

It took police more than nine hours to complete a floor-by-floor search of The Bow tower for a man allegedly armed with a knife and uttering threats of terrorism.

Police evacuated the building, which is the tallest in Western Canada, after receiving a call around 4 p.m. Tuesday saying the man was in the women's bathroom on the building's 55th floor.

One of the bathroom stalls was locked when security guards went into the washroom. They told police they could see shoes under the stall door and made contact, but there was no response.

Security forcibly entered the stall, but the man rushed past the guards.

"He was uttering some threats and he did advise security to call police and call SWAT," said police spokesperson Kevin Brookwell. "At that time, the security guard indicated that he believed he saw the butt end of knife. He also recalled the individual saying something about terrorist or terrorism. So a 911 call was made."

There was also the smell of smoke in the washroom, so the Calgary Fire Department was called. Police say around that time the fire alarm was pulled, and officials are not sure who activated it.

Roughly 4,000 people were forced to leave the building.

No suspicious packages found

"We didn't have resourcing in place at the time for the flood of individuals that were coming out of the building, so we were not able to 100 per cent confirm the individual we were looking for — who was a Caucasian male about 30-years-old, scruffy hair, wearing black hoodie, red ball cap and blue jeans — ... was still either in the building or had previously left the building," said Brookwell. 

"So we made the decision that we were going search the entire building, given what was said [and] potentially the impact it could have."

Police searched the tower until 1:30 a.m. Wednesday but did not find the suspect or any suspicious packages.

"We are taking this matter very, very seriously. We did what we needed to do to ensure the safety of that building and the safety of the folks that work in that building, so it did take a little bit of time," said Brookwell. 

On Wednesday officials said CCTV footage led them to believe they knew who they were looking for. The suspect was a man with mental health issues who was known to police.

Police said they didn't think the man was any risk to the public.

Police still don't know how the man initially got past Bow tower security at the front entrance.

Brookwell said the operation — which included tactical and canine units — was expensive but police had to respond.