Andrew Holt/Getty ImagesThere is no evidence to suggest the substation fire that shut down Heathrow airport was suspicious, police say.
The fire, which broke out in Hayes on Friday, resulted in the closure of Britain's busiest airport for a day.
Counter-terrorism officers were briefly drafted in to investigate. However, the Metropolitan Police said on Tuesday that the incident was no longer being treated as a "potentially criminal matter".
Officers continue to support the London Fire Brigade, the National Grid and "other partners" to investigate the cause of the blaze.
Police say they will consider "any relevant new information or evidence" should any come to light.
While investigations into the cause of the blaze are ongoing, it has been reported the fire started in a transformer within the substation in Hayes around midnight.
The blaze caused a temporary power outage at the airport that led to thousands of cancelled flights on Friday and stranded passengers across the world.
Nearly 1,400 flights were disrupted by the airport's closure, according to air traffic website flightradar24.com. Around 120 flights were diverted elsewhere.
The incident also disrupted the supply of millions of pounds of goods that go through the airport.
More than 63,000 homes lost power in the outage caused by the fire, according to energy supplier Scottish and Southern Electricity Networks.
Operations at the airport were halted for 18 hours on Friday, with bosses saying it was "open and fully operational" by Saturday morning.
However, the chaos has raised concerns about the resilience of the major transport hub - and brought into question the UK's energy resilience more broadly.
Energy Secretary Ed Miliband has called for lessons to be learned from the incident.
Substations, operated by the National Grid, are designed to produce, convert, and distribute electricity at suitable voltage levels.
Heathrow uses three electricity substations.
The airport also has emergency back-up power supplies of its own, which use diesel generators and batteries. These supplies only keep crucial safety systems running, such as landing equipment and runway lights.
Heathrow's chief executive has said the shutdown was caused not by a lack of power, but by the time it took to switch from the damaged substation to the energy provided by the two other operational substations.
Airport bosses have been criticised for their decision to shut Heathrow down following the claim that it had enough power despite the substation fire.
Prime Minister Sir Keir Starmer told the BBC he was "deeply concerned" by the closure of the airport, adding "there are questions" for managers to answer.
The airport has said "lessons can and will be learned".
