Detective Superintendent Jason Murphy, who is leading the inquiry, described the murder as "senseless and appalling beyond belief". I call on anyone with information to come forward to the PSNI. A self-described "ceasefire baby", too young to have her own memories of the 1970s and 1980s, she brought curiosity and courage to her explorations of Northern Ireland's history.
The UK Prime minister and the leaders of political parties in the North have also condemned the violence and paid tribute to the award-winning journalist.
"We can not allow those who want to propagate violence, fear and hate to drag us back to the past", Irish Prime Minister Leo Varadkar said in a statement.
She tweeted: "I was standing beside this young woman when she fell beside a police Land Rover tonight in Creggan #Derry".
"Our hopes and dreams, and all of her wonderful potential, was snuffed out by a single barbaric act".
I am asking people to do the right thing for Lyra McKee, for her family and for the city of Derry/Londonderry and help us stop this madness.
A murder investigation has been launched but there have been no arrests yet.
The unrest comes ahead of an Easter weekend which will see republicans mark 103 years since the 1916 Easter Rising as well as the 70th anniversary April 18, 1949 - when Ireland officially became a Republic.
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He added: "Our assessment at this time is that the New IRA are most likely to be the ones behind this and that forms our primary line of enquiry".
"The Belfast-based investigative journalist has crowd-funded money to finance research for her first book about the murder of a Northern Irish politician that dates back more than 30 years", her Forbes citation said. Members of the New IRA took responsibility for a January auto bombing in Derry, and for five package bombs that had been sent to addresses in London and Glasgow in March. It is regarded as the largest of the splinter dissident groups still operating and has been linked to several other killings in the past decade.
Hamilton said police believe the shooting was carried out by dissident Republicans, namely the New Irish Republican Army.
In the video, a figure near the gunman appears to crouch and pick something up from the spot where the shots were fired, seemingly confirming detectives' theory that more than one person was involved.
The IRA sought Northern Ireland's integration into the Republic of Ireland through violence.
Prime Minister Theresa May and Taoiseach Leo Varadkar condemned the killing, while former USA president Bill Clinton tweeted: "Heartbroken by the murder of Lyra McKee and the violence in Derry".
There has been an increase in tensions in Northern Ireland in recent months with sporadic violence, much of it focused in Londonderry, also known as Derry. But some groups do not accept the agreement, and one such group is likely responsible for McKee's death.