London (CNN) -- A British girl who survived the execution-style killing of her father, mother and grandmother in a normally idyllic region of the Alps last week returned to Britain on Friday, authorities said.
Zainab al-Hilli, 7, left France in the morning and arrived in the UK, Surrey police said.
As the only survivor, she could be a key witness who saw who carried out the shooting, according to the French prosecutor leading the investigation.
She spent several days in a medically induced coma, and as of Wednesday, she remained too badly injured to be questioned, Eric Maillaud said.
She was beaten and shot in the attack, and rescued by a British cyclist who came upon the scene on September 5.
The cyclist, former Royal Air Force pilot Brett Martin, helped the injured girl and called for help.
In an interview with BBC News, he recounted the shocking scene he stumbled across as he went on a regular bike ride in a national park.
Martin said what he found in the parking lot -- three people shot dead in a car and a fourth on the ground nearby -- was like "a Hollywood scene."
His account came as French prosecutor Maillaud and investigating Judge Michel Mollin were in Britain seeking new leads in the case.
They met with Surrey Police, whose officers have been helping French investigators to search the home of two of the victims, Saad and Ikbal al-Hilli, in Claygate town, Surrey county.
Maillaud said it was "highly likely that the reasons and causes for (the crime) have their origin in this country."
"Our presence here does not mean that there are problems between the two jurisdictions," Maillaud said. "We wish to reinforce our cooperation and understanding with hope to reach a conclusion to these horrible murders."
Surrey Police will do all they can to assist the French-led investigation, said Rob Price, the assistant chief constable.
Maillaud said Wednesday that investigators were focused on three main areas: Saad al-Hilli's job, his links to his native Iraq and a reported family dispute over money involving his brother.
Born in Baghdad in 1962, Saad al-Hilli was a naturalized British citizen who had lived in the United Kingdom for decades. He was an engineer working at Surrey Satellite Technology, a high-tech company owned by EADS, an aerospace corporation that builds satellites.
Authorities have been tight-lipped about possible perpetrators and motives in the attack, although speculation has been rife in the British media.
Martin's account to the BBC shed fresh light on the grisly scene that confronted French police near Lake Annecy after the alarm was raised last Wednesday.
The first thing he saw as he cycled up the mountain road near the village of Chevaline was the bike of a French cyclist on the ground, Martin said.
Then he spotted the wounded girl, whose parents and grandmother were subsequently identified as those shot to death in the car.
She was stumbling and falling over, and at first he thought she was playing, Martin told the BBC. Then he realized she was injured and put her in a recovery position as she slipped in and out of consciousness.
The car engine was still revving and wheels spinning, he said, making him fear it could move and harm the girl and the cyclist whose bike he had seen on the ground.
"At first I thought, there's been a terrible accident between a cyclist and a car, because there was a cyclist on the ground more or less in front of the car. But there were things that didn't quite match, because the cyclist's bike wasn't beside him, so as the minutes went on, I started to change my opinion," he told the BBC.
He pushed in one of the car windows, which had bullet holes in it, to turn off the engine -- and saw the bodies inside.
"I've never seen people who've been shot before for real ... but it seemed to me just like a Hollywood scene, and if someone had said 'cut' and everybody got up and walked away, that would have been it. But unfortunately, it was real life. ... It became quite obvious now, taking stock, that it was a gun crime," he said.
Realizing that whoever was responsible could still be in the area, Martin became increasingly anxious but faced a dilemma.
He had no cell phone signal to call emergency services, but the girl appeared too badly injured for him to carry her down the mountain. Martin decided to leave her in a safe position and set off back down the road on his bike to summon help.
He managed to flag down a car and asked the French motorist to call for help, before returning to the scene to check on the girl, who was then unconscious.
He added that he was not surprised that French police had failed to spot a second child, a 4-year-old girl, hiding in the back of the car under her dead mother's legs, for nearly eight hours.
The girl, who has been reunited with other family members, told investigators she heard noise but saw nothing.
Martin, from Sussex in southern England, went back to France on Wednesday to retrace his route and see if new recollections came to mind.
The pilot, who now works in civil aviation and has a family business in Annecy, had given a detailed statement to police immediately after the shooting, including details of vehicles that passed him on the road, he said.
CNN's Kirsten Dewar, Laura Smith-Spark and Stephanie Halasz contributed to this report.