Louise Parry & David Freezer
BBC News, Essex
UKNIP
Eight Air Accident Investigation Inspectors are now at the site in Essex
A pilot and co-pilot who died along with two others when a small plane crashed at London Southend Airport were Dutch nationals, it has been confirmed.
The airport has been closed since emergency services were called to the incident involving a Beech B200 Super King shortly before 16:00 BST on Sunday.
Witnesses described a "fireball" type explosion soon after the Netherlands-bound light aircraft had taken off.
A Dutch foreign ministry spokesperson confirmed the nationality of both the pilot and co-pilot and said it was "in touch with the families and offering consular support".
The names of the four who died have not been released but officers are now working to confirm their identities.
Ch Supt Morgan Cronin said the victims would be "treated with the utmost respect and dignity".
The Civil Aviation Authority (CAA) has put in place a significant air exclusion zone surrounding the crash site.
All flights to and from the airport on Monday have been rerouted, with passengers advised to check with their airline for advice.
Aerial video showed the plane crash site and wreckage
The plane was operated by Zeusch Aviation, based at Lelystad Airport in the Netherlands, which has confirmed it is "actively supporting the authorities with the investigation".
The aircraft had flown from the Greek capital Athens to Pula in Croatia on Sunday before heading to Southend. It was due to return to Lelystad on Sunday evening.
BBC reporter Sofia Bettia has been at the headquarters of Zeusch Aviation – a privately owned company operating 14 aircraft.
"This is a small airport in the Netherlands, on an island to the east of Amsterdam - and today, it is almost deserted.
"We just saw airport security staff lowering the Dutch flag to half mast at the entrance of the airport.
"Zeusch specialises in medical evacuation flights, private charters, and aerial filming for various events.
"We were able to speak to a pilot from another airline. He told us he's worried that one of his colleagues may be among the victims, as many of the staff here know one another."
A timeline of how the light aircraft crash unfolded at London Southend Airport
Detectives and forensic teams are working in parallel with the Air Accidents Investigation Branch (AAIB), the Royal Air Force, Essex County Fire and Rescue Service and London Southend Airport to work out what happened.
Speaking at the press conference, Lisa Fitzsimons, a senior inspector at the AAIB, said it was "too early to speculate on what caused this tragic accident".
"If there are safety lessons that can be learned, we will make recommendations that will aim to prevent a similar accident occurring," she said.
Eight of its inspectors are now at the crash site.
John Johnson, 40, from Billericay, Essex, witnessed the crash while watching planes with his wife and children.
"You could see [the pilots] smiling and we all kind of smiled back.
"It took off in probably three or four seconds. It started to bank heavily to its left," he told the BBC.
Then within a few seconds, "the aircraft almost inverted and hit the ground. There was a huge fireball," he said.
Smoke rises at Southend Airport after the plane crash
A London Southend Airport spokesperson said: "Our thoughts are with those affected by [Sunday's] events and all passengers impacted by this disruption.
"We will restart flight operations as soon as possible and will continue to update the public on developments."