LONDON, England (CNN) -- Two men accused of terror attacks in Britain planned to kill themselves in a suicide bombing, sources told CNN Wednesday after police found an apparent suicide note.
Images from Glasgow Airport show the vehicle ablaze next to the terminal building.
Investigators found a suicide note linked to the Glasgow, Scotland, attack, sources close to the investigation told CNN Wednesday.
The letter indicates the men intended to detonate an explosive device in the the sport-utility vehicle while still inside the vehicle, the sources said.
Police also say they believe the men -- identified as doctors Khalid Ahmed and Bilal Abdulla -- were behind two car bombing attempts in London a day earlier, the sources said.
They allege the men parked two cars packed with explosives in Central London then drove six hours north to Scotland. Watch as police probe alleged sleeper cell »
Britain lowered its threat level from its highest level "critical" to "severe" Wednesday, saying the threat of a terrorist attack was no longer imminent.
British authorities have detained eight people in connection with the terror probe and believe they have all of those responsible for the attacks in custody.
It is unclear whether the attack at Glasgow Airport -- which happened a day after the first car bomb in London was discovered -- was a last-minute decision or designed to follow the London car bombs.
One of the suspects, identified by sources as Dr. Khalid Ahmed, was hospitalized for critical burns from the attempted suicide bombing at the airport.
Another suspect, identified as Bilal Abdulla, a 27-year-old Iraqi doctor, is in police custody along with six other suspects. All eight have links to the medical profession, according to sources with knowledge of the investigation.
British Police have not named any of the suspects in their custody, citing the ongoing investigation, but sources close to the investigation have confirmed the identity of the suspects. See who the suspects are »
Investigators believe the plot may have been hatched before the suspects arrived in Britain in recent years.
U.S. officials told CNN they believe some of the suspects were recruited by al Qaeda while they were living in the Middle East.
A British Anglican cleric working in Baghdad said a man he met in Jordan in April issued a disturbing threat that may have been a portent of the bombing attempts.
"Those who cure you will kill you," the man told Canon Andrew White, who spoke to CNN on Wednesday.
And according to a report in a British newspaper, The Daily Telegraph, some of the suspects had been tracked by MI5, the British intelligence service, before the attempted attacks.
In his first question-and-answer session since he took office, British Prime Minister Gordon Brown Wednesday vowed to take measures to increase security in the wake of the attempted bombings, which happened days after he became prime minister.
In addition to expanding the watch list of potential terrorists, Brown said background checks will be stepped up and British authorities will keep a closer eye on how it recruits doctors from other countries.
"I've asked Lord [Alan] West, the new terrorism minister, to conduct an immediate review as to what arrangements we must make in relation to recruitment to the NHS [National Health Service] because of what we know has happened over the last few days," Brown told the House of Commons.
Brown, who was born in Glasgow, took office just two days before the London bombs were found. See timeline of investigation »
Australian connection alleged
Australian authorities picked up one of the suspects, an Indian national and doctor, on Monday trying to leave Brisbane, Australia, reportedly on a one-way flight to India. He has been identified by sources close to the investigation as Dr. Mohammed Haneef, 27.
Prime Minister John Howard denied there was any evidence the terror plot extended to Australian soil.
"There is nothing that has happened in the UK in and of itself that suggests there is a greater danger of there being any terrorist incident in Australia," Howard said.
Haneef had a temporary visa and worked at a hospital in the Brisbane area, where his office was searched by police, authorities said.
Haneef had previously worked at Halton General Hospital near Liverpool, England, where another doctor in custody -- identified by British media as Sabeel Ahmed -- was employed, according to hospital and police officials.
Both Ahmed and Haneef graduated from Rajiv Gandhi University of Health Services in India, British media reported.
Haneef and another doctor had been recruited to work in Brisbane last year through an ad in the British Medical Journal, according to Queensland Premier Peter Beattie. E-mail to a friend
CNN's Kelli Arena, Andrew Carey, Matthew Chance, Paula Newton, Cal Perry, Nic Robertson and Alphonso Van Marsh contributed to this report.