There were unconfirmed reports the gunman then hijacked or stole a car and drove to the Parliament buildings, where at 9.54am, eyewitnesses saw armed police pursuing him.
"I couldn't hear the shots but we had word come through very quickly and went into a lockdown procedure," Mr Downey said.
The lockdown initially meant access to and from the CBC building was prohibited.
But with so many shots fired, nobody knew if the gunman had accomplices, so the CBC building entered a full lockdown.
Stairwells were out of bounds, elevators shut and staff security badges deactivated, as staff were told to stay away from windows.
Mr Downey said there were armed men on neighbouring rooftops but it was impossible to know who they were.
"The police and military at this point were putting snipers on our building and on all the buildings around us. But ... we didn't know if they were gunmen initially, or whose side they were on."
Meanwhile, the gunman rampaged in a hallway in Parliament until 56-year-old former Mountie Kevin Vickers shot him dead.
Outside, confusion remained, amplified by the novelty of an armed fanatic in normally peaceful Ottawa.
"People are really shocked," Mr Downey said. "It's probably similar to how you guys would be doing if it happened in Wellington."
A partial lift of the lockdown began as the business day drew to a close. Mr Downey was relieved to make it home to his wife and children in their home, about 20km from the city.
By nightfall, Mr Downey said shock was possibly wearing off for Ottawans, and another emotion replacing it. "I think anger would be the most common term I've heard today."
Mr Downey said Canada, like New Zealand, was proud of being a free and open society. He expected some of those freedoms to now be curtailed.
"People are understandably and rightly so asking some tough questions about how the gunman got right into the Parliament building. How did he get that far?" The attack followed an attack in Montreal just two days earlier. In both cases, Canadian citizens who converted to strands of fundamentalist Islam were believed to be the culprits.
Prime Minister John Key said the chances of a similar event occurring in Wellington could not be ruled out.
Parliamentary Service has beefed up security at the Beehive and surrounding precinct by restricting all access except for two doors. Officials said the measure was likely to be in place for several weeks.
- additional reporting: Isaac Davison.
