A man thought to have been the victim of an exploding mobile phone has left investigators baffled after engineers examined the device and gave it the all clear.

Luis Picaso, 59, is in a critical condition with 50 per cent second- and third-degree burns to his upper body, back, right arm and right leg after being found in his hotel room in Vallejo, California.

The cause of the fire was assumed to be his mobile phone, which was still in his pocket where the fire started.

But engineers from Nokia have flown to California to examine the 2125i handset and gave it the all clear and were even able to switch it on. Tests were carried out in the presence of the fire investigator and seem to show Nokia in the clear.

"A cell phone cannot create a spark," Nokia engineer Keith Nowak told the San José Mercury Herald.

"The most likely way a spark can happen is from a short in a battery, and if that happened, the phone would not work anymore."

Picaso has just received a skin graft, the first of many, and is unable to speak, but his attorney Vance Owen has disputed Nokia's findings.

Owen told the Mercury Herald that, since his client did not smoke, the mobile phone was the logical culprit. He added that getting Nokia to determine whether its own product caused the blaze was "like asking an arsonist if he started the fire".

While there have been instances of mobile phones overheating and catching fire, the usual culprit is third-party batteries with faulty power management controllers.