"It's the moment that everyone who works in or around Parliament is told to prepare for.
When your office is one of the most important and high profile buildings in the world, that makes it a prime target.
But no matter how many times a policeman stops you to check your pass, or one of the dozens of green screens dotted around Westminster Palace reminds you the threat level remains at "severe", you never quite believe that anything serious will ever happen. Could ever happen.
The first sign that something was wrong was the sound of raised voices in the screening area at the entrance to Portcullis House.
Walking past the floor-to-ceiling glass frontage, it was clear some form of scuffle was breaking out.
MPs, political aides and reporters paused on their way to and from meetings and debates to watch.
Then the shouting began.
Police and security officers were yelling at us to "get back! Stay back!". The usually orderly queue of people waiting patiently to step through the metal detectors and collect their bags from the X-ray machine was replaced by a swarm of people being ushered through the doors to safety.
Reaction was slow at first, as those going about their business looked at each other as if to say "is this really happening?", " did we really just overhear the words 'there is a person down' on that police radio?"
But as the shouting continued and policemen began sprinting for the entrance, the panic set in.
Someone claimed they heard gunshots. Another that a policeman had been stabbed. Was there someone in the building? A bomb?
That's when the first tweets started dropping. It wasn't just one incident, it was multiple. Colleagues in the lobby were posting reports of chaos on Parliament square as tourists fled to escape an unknown threat. One fellow lobby reporter had seen a car ramming the gates, another saw a man lunge at police, before shots were fired and he crumpled to the ground
As the facts built up, the reality of the situation sank in. MPs, caterers, cleaners and journalists alike were calling loved ones, reassuring bosses, and scouring social media for details.
In the ensuing lockdown, we learnt piecemeal of the heroes who put their life on the line to protect the men and women who work in Parliament. We learnt of the innocent people who lost their lives because they strayed unwittingly into the path of hate. And we learnt of the politicians who put thoughts of their own safety aside to try and save a wounded policeman.
We also learnt how quickly our sense of security and normalcy can crumble when an attack like this reveals how vulnerable even our most cherished institutions can be."