Though the man is not thought to be on any terrorist "worry lists", and is not a known criminal, sources admitted yesterday they needed to check other possibly worrying items in his possession, which they refused to discuss.
The man, who was carrying a Venezuelan passport, is a Muslim and it is understood documents were found in his luggage. He was not, however, carrying a "hard copy" of the Koran, as was first thought, police said yesterday.
Detectives say they have no firm idea why he was carrying the weapon and are cautious of jumping to conclusions in the current climate. "We don't know if we've got an oddball with a grenade on a plane, which is bad enough, or some thing more sinister," said a source. "It could be several days before we bottom this out."
Police do not believe the man is linked to the ongoing security alert at Heathrow, which began with intelligence that al-Qaida sympathisers in the UK had Sam-7 anti-aircraft missiles at their disposal.
The Gatwick suspect, who is being held at in London, was arrested by customs officers on Thursday, shortly after disembarking from a British Airways flight from Bogota.
Whitehall sources described finding the grenade as "lucky" rather than the result of any intelligence-led operation.
Six other people were being questioned yesterday under the Terrorism Act 2000, and terminal 2 at Heathrow was temporarily evacuated in what proved another edgy 24 hours at the UK's major airports.
Four men in their 20s were arrested on Thursday morning close to Heathrow when police manning a roadblock stopped a car in Langley, near Slough, Berkshire. The suspects were detained for immigration offences.
Two men were also arrested on Thursday near the perimeter of Leeds-Bradford airport. Police sources described the arrests as "precautionary".
Heathrow's terminal 2 was evacuated for an hour as police investigated a suspect package.
A spokeswoman said the airport had only followed "agreed protocols" when the bag was not collected. As the evacuation got under way, passengers were ushered on board aircraft and locked in.
A British Airways executive at Heathrow described the last few days as "chaos".
"All the operations here are very carefully dovetailed," he said. "You've got an aircraft coming in from Frankfurt and just an hour later, the crew will go to Oslo and the plane goes to Edinburgh. Once you get out of schedule, you get utter mayhem."
Airline staff at Heathrow are becoming increasingly concerned at the security crisis. One source said: "We're conscious that we all need jobs and the passengers are just vanishing."
