"We certainly believe there was more than one person who was involved in this last night. Obviously only one person pulled the trigger but there was more than one person," he said.
"There was certainly more than one person involved in the murder."
Asked about speculation one of those who carried out the killing was a teenage boy, Deputy Chief Constable Martin said it was "too early" to speculate on the age of the gunman.
He also defended the police searches that led to the disorder, stating it was designed to prevent future attacks by dissident republicans.
"We carried out that search activity last night because we believed that there were people in this city who, over the weekend, were going to mount murderous attacks on the police and perhaps even other events, such as that which we saw with the bomb attack in January on Bishop's Street," he said.
"Before I came here today, I read the intelligence that led to that search activity. I wanted to come and talk to the people of this city and say that I read it myself, I convinced myself of the urgency to carry out that search activity- and I am convinced."
"When we engage in that type of activity we don't do it lightly, we understand the impact it can have on a community."
The Mayor of Derry City and Strabane, John Boyle, said the murder was "heinous in the extreme" and paid tribute to Ms McKee.
"I just want to reflect on the fact that I personally knew Lyra McKee, I have known her since she was 16-years-old," he said.
"She was bright, she was warm, she was witty, but most of all she was an outstanding individual, a great friend to so, so many people in this city in the short time that she was with us.
"We are broken-hearted for her family at their loss and, again, I want to just extend our very deepest sympathies to them.
"There's an awful lot of anger in this city today. It has to be said again, this was not done in the name of the people of this city. Those responsible need to understand that. I can't put it any stronger than that.
"Not only that, but the people of Creggan absolutely abhor what happened last night. Lyra was one of us."