Queen Elizabeth is sending a private message of condolence to the widow of former IRA commander Martin McGuinness.
The British monarch met and shook hands with Mr McGuinness at a charity event in Belfast in June 2012 - an event that would once have been unimaginable.
Buckingham Palace indicated that the queen would be contacting his wife, Bernie, following his death.
The handshake lasted mere seconds, but it was a moment of deep significance.
"I feel so privileged to have witnessed that significant and historic moment," said former PA photographer Paul Faith, who was there.
Mr Faith was the only photographer allowed access to captuure the handshake.
"I remember that Martin appeared very relaxed and confident and happy to stand beside the queen," he said.
"The significance of it for me didn't really sink in until the next day when I saw the pictures in the papers.
"Every front page was carrying the photograph, and it hit me that this was going to be a historic image.
"The day after the handshake, I was sitting in the press room at the Irish Open and Martin McGuinness came in with Peter Robinson.
"He came over to me and shook my hand and said, 'You've made me famous again'.
"He then asked for a photograph of him and I shaking hands.
"I also took the photograph of him and Ian Paisley that led to them being called the Chuckle Brothers. He later signed that one for me and said, 'So, Paul, you're the one who turned me into a Chuckle Brother. Well done'."
The day after shaking hands with the queen, Mr McGuinness spoke about its "momentous and historical" significance.
He said that the meeting had the potential to define "a new relationship between Britain and Ireland and between the Irish people themselves".
In a speech in Westminster, Mr McGuinness said the handshake "was in a very pointed, deliberate and symbolic way offering the hand of friendship to unionists through the person of Queen Elizabeth, for whom many unionists have a deep affinity".
"It is an offer I hope many will accept in the same spirit as it was offered," he said.
Former US Secretary of State Hillary Clinton has described the handshake as "the most remarkable sign of change yet" in the Northern Ireland peace process.
A few years later, Mr McGuinness shook hands with Prince Charles when he visited a Catholic church in East Belfast.
A few years later, Mr McGuinness paid tribute to the queen for meeting him when he appeared in a BBC documentary.
"I liked her courage in agreeing to meet with me. I liked the engagements that I've had with her," he said.
"There's nothing I have seen in my engagements with her that this is someone I should dislike - I like her."