A teenage boy arrested on suspicion of murdering two British holidaymakers in Florida had already been detained by police for aggravated assault with a handgun, police said.
Shawn Tyson, a 16-year-old who lives close to the murder scene, is being held in connection with the killing of James Cooper, 25, and James Kouzaris, 24.
The bullet-ridden bodies of the two tourists were discovered on a street in Sarasota's run-down district of Newtown at about 3am on Saturday.
Tyson was previously arrested on April 7, reportedly for firing into a car, Sarasota Police Department said.
Captain Paul Sutton said the teenager would be charged as an adult over the two murders, which means he could face the death penalty if convicted.
Meanwhile detectives were today investigating what led Mr Cooper and Mr Kouzaris from a night out in the centre of the city to the deprived, crime-ridden district where they were shot dead.
It was "very unusual", they said, to find tourists in the neighbourhood - a housing project about 12 miles away from the upmarket island city of Longboat Key where the university friends were staying.
Mr Kouzaris, from Northampton, was on a three-week holiday in Florida with Mr Cooper and his family, from Warwick.
The pair had been on a night out visiting bars in downtown Sarasota on the Friday evening, before being found dead hours later in a narrow one-way street 20 blocks away.
Mr Sutton said detectives were "examining all theories" and "keeping an open mind" as to how and why the friends came to be in the "no-go" area.
He said the Britons may have befriended someone who gave them a lift, could have got a cab which detectives have not yet traced, or may have walked.
"Anything you could imagine is a possibility," he said.
Sarasota police chief Mikel Hollaway said detectives had their "suspicions (about what they were doing there) but at this time it would be unfair to state those".
He said he believed the victims were there of their own volition.
Mr Sutton said there was no known link between the suspect and the victims.
"It is very unusual to find tourists or visitors in this area. It is a residential neighbourhood with no shops and no bars. We do not know what brought them here at 3am," he said.
Asked whether there could be more arrests in the case, Mr Sutton said: "We're looking at the possibility. More than one person ran when the shots were fired.
"Are other people involved or is it people who just happened to be there?"
Police confirmed the two Britons were not carrying any drugs but would not say whether they had any weapons or an unusually large amount of money.
Early reports that the men had been murdered with a machine gun were also quashed by Mr Sutton, but it is believed they were each shot a number of times.
Residents in the Newtown area speculated that the men could have been lured into an ambush by a gang of masked gunmen.
Sonja Seymour, 31, who lives on the street where the men's bodies were found, said in reports: "I heard that when they arrived here there were already some people waiting for them. They were wearing masks.
"The men ran away and they were shot. I saw one of them lying in the street on his back with his arms outstretched.
"The other man was across the road lying on his back. They were not moving.
"The ambulance arrived and tried to work on the two men but they couldn't do anything."
Mr Cooper had spent Thursday night with American friend Gina Cross, 31. The pair had kept in constant touch since meeting two years ago.
Ms Cross, from Sarasota, told the Daily Mail: "I don't know how they ended up where they were found.
"It's a very bad neighbourhood - somewhere where I would not even drive."
Mr Sutton said Tyson had not yet been charged, but should be within 21 days.
Mr Kouzaris, a popular and successful amateur rugby player, spent several months travelling in South America before his death, visiting Ecuador, Argentina, Brazil, Colombia and Bolivia.
In a message on his Facebook page, he spoke of looking forward to returning to Britain on March 27, writing: "Back to normality soon."
Mr Cooper, who worked as a tennis coach, replied: "U have got to be kiddin, ur back a week and then we go off to america."
Mr Kouzaris's parents described him as a "wonderful son" in a brief statement released through Northamptonshire Police.
They said: "We loved him so much and we can't believe he has gone. We are absolutely devastated and in a state of complete shock."
His sister Emily added in an emotional message on Facebook: "My brother was a legend and he will be missed and loved by many many people."
Mr Cooper represented Warwickshire at tennis and once played Andy Murray in a junior tournament.
James Roe, his friend and former coach, said: "He started off as a pupil but he became a really good friend. He was a normal guy, he had a very responsible job as a coach.
"He was an only child and was the apple of his mum and dad's eye. He did really well at school and then at university. He was a model student and a cracking tennis player.
"It is a sad loss because he had a flourishing tennis career ahead of him. He was really going places."
Mr Cooper studied at Sheffield University's management school before becoming a full-time tennis coach for Warwick University-based company inspire2coach.
He had just been promoted to a head coach position and was due to start his new role after returning from holiday.
Mr Kouzaris attended Northampton School for Boys, where he was captain of the school's rugby team, before completing a four-year town planning degree at Sheffield.
He worked for Northamptonshire County Council as part of a team setting up a programme to provide healthy food for schools.
Mr Kouzaris also captained a team at the Old Northamptonians Rugby Club from the age of around nine until he went to university.