And her family, who lost Ms Cox when she was murdered moments before she arrived at a constituency surgery in 2016, knew the pain that Sir David’s loved-ones will have to endure.
Asked how she felt, Ms Leadbeater said: “Totally shocked by what has happened to think that something so horrific could happen again to another MP, to another family.
“And scared and frightened – a real rollercoaster of emotions.
“My phone started going straight away, my mum and dad, my partner, my friends, ‘Are you OK?’ and I was OK, I was visiting a school.
“But the shock and the feelings for us as a family, obviously what we went through and another family are going through that again, it’s horrific.
“It’s hard to put into words how that feels for me.
“The main people I am thinking about are David’s family, his friends and the community he represents and has represented for such a long time.
“I think that’s the thing that people need to understand, it’s about a lot of people whose lives have changed forever today.
“People still come up to me and talk to me about where they were when Jo was killed.
“Their lives have just been changed.”
She said the role of an MP held risks.
“It’s so hard because you have a job to do,” she said.
“I find myself now working as a politician and trying to do good things for people and it’s really important you get good people in public life, but this is the risk we are all taking and so many MPs will be scared by this.
“My partner came home and said ‘I don’t want you to do it any more’ because the next time that phone goes, it could be a different conversation.
“There are so many layers to this.
“At the heart of it are David’s family and friends.
“I know for them now that their lives will never be the same again, they will think about this every single day for the rest of their lives.
“Even David’s staff – so many other people today will have been out there trying to do the right thing, trying to do a really important job in public life, and this happens.
“I cannot believe that this has happened.
“It feels very raw for me.
“I know from messages I have received from politicians across the political spectrum, for them it is incredibly raw.
“The main people on my mind are David’s family and friends and I know the rollercoaster that they will now be on.
“There’s so little that you can say because it’s happened.”