The burned-out vehicle has now been removed from Glasgow Airport
Deputy Assistant Commissioner Peter Clarke, the UK's top counter-terrorism officer, has said the inquiry is "extremely fast-moving" with "new information coming to light hour by hour".
What is involved in the operation?
ARRESTED SUSPECTS
Police are holding eight people in connection with the attempted car bombings in London's West End and at Glasgow airport.
One man - who suffered severe burns in the Glasgow airport attack in which a burning car was driven into the airport entrance - remains in a critical condition and under armed police guard at Royal Alexandra Hospital in Paisley.
A second man was also arrested at the scene. He has been taken to London's Paddington Green police station for questioning.
Two further arrests were made on the M6 in Cheshire, with both detainees - a man and a woman - being questioned at the London police station.
Another man, arrested in Liverpool, is also in custody at Paddington Green.
BBC News has learned that these five people are believed to be of Middle Eastern nationalities.
Two more men were arrested at residences of the Royal Alexandra Hospital in Paisley on Sunday night.
An eighth person was arrested in Australia while trying to board a flight to India at Brisbane airport, on Monday.
Australian police are also interviewing a second doctor. They were acting on information from the UK authorities, Australian Prime Minister John Howard said.
Warrants have been issued which allow police to continue questioning the three people arrested in England until Saturday.
Police can detain people without charge for up to 28 days under the Terrorism Act 2006, although extensions must be sought from a judge from the 48-hour stage onwards.
In other circumstances suspects can be held for just 36 hours, although extensions can be sought up to a total of 96 hours.
EXAMINING VEHICLES
Forensic examinations are continuing of the two Mercedes cars and the Jeep Cherokee involved in the attempted bombings.
BBC correspondent Daniel Sandford said: "The astonishing speed of this investigation has been due to the goldmine of evidence in the car bombs found undetonated in London."
Deputy Assistant Commissioner Peter Clarke, of the Metropolitan Police, said study of the vehicles was proving "extremely valuable".
The first Mercedes, parked by the busy Tiger, Tiger nightclub in Haymarket, contained gas cylinders, 60 litres of petrol and nails. An ambulance crew alerted police when they saw vapour coming out of the car.
The second device, which also included petrol, cylinders and nails, was discovered in a Park Lane car compound after being towed away.
It had been illegally parked near Trafalgar Square, just a few minutes' walk from the first vehicle.
The BBC understands both devices were linked to mobile phones, a common trigger technique in improvised bombs. It is thought that this remote trigger failed.
Both cars have been taken to a maximum security laboratory in Kent for examination.
Strathclyde Police issued the registration number of the Jeep Cherokee - L808 RDT - and appealed for information about sightings of the vehicle in recent weeks. It crashed, on fire, into Glasgow Airport's terminal buildings on Saturday.
Police conducted two controlled explosions on two vehicles at the Royal Alexandra Hospital on Monday.
Three controlled explosions were carried out on a car at a Glasgow mosque on Tuesday morning.
SEARCHING ADDRESSES
The investigation has involved searches of addresses in the village of Houston in Renfrewshire, Liverpool and Newcastle-under-Lyme, Staffordshire.
Police have also been searching premises within the grounds of the Royal Alexandra Hospital.
The home secretary said 19 locations had been searched by police investigating the attempted attacks.
Australian police are searching sites at the Gold Coast Hospital in Southport, eastern Queensland, plus other locations across the state.
UTILISING TECHNOLOGY
Metropolitan Police officers have obtained clues about the London attack attempts from CCTV and congestion charge cameras.
Deputy Assistant Commissioner Clarke says the investigation involves studying thousands of hours of CCTV footage to help "piece together the events of the past few days" - a process he says will take "many weeks" to complete.
Automatic number-plate recognition systems have been used to track movements by the vehicles and mobile phone records have helped police swiftly locate some of the people currently in custody, our correspondent says.
New Scotland Yard, the headquarters of the Metropolitan Police Force
A suspicious bag has undergone a controlled explosion outside a London Underground station, as has a car outside a mosque in Glasgow, Scotland.

These police actions come in the wake of failed terror attacks involving car bombs in London and at Glasgow International Airport on 29th and 30th June.  

The terror threat level in the UK remains ''critical'', indicating that an attack could be expected imminently and high security is in evidence on mass transport routes, while the police continue their investigations.  Earlier this morning, a controlled explosion took place on a suspicious package discovered outside Hammersmith Station on the London Underground system. The scare led to the closure of Hammersmith and Barons Court stations for one hour. 

Two other stations, Tower Hill and East Ham, were closed in similar circumstances. All four stations have now been reopened.

Sky news reports that three controlled explosions took place on a car outside a mosque in Glasgow, in order to allow police to gain entry to it. The mosque itself is not being linked to the terror attacks.

The Muslim Council of Britain has condemned the attacks, and has urged Muslims throughout the country to assist in police investigations.

The police force is continuing its investigations throughout the country, with an open appeal to the country for vigilance in reporting anything suspicious.