Washed-up drugs smuggler John Gilligan "urgently needed to go to the toilet" when he saw Spanish police dig up the gun they claimed could be the Veronica Guerin murder weapon.
The Dublin gangster had to rush to the bathroom after losing control of his bowels.
Sources described his reaction after police found the Colt Python .357 Magnum hidden under gravel in the garden of his Costa Blanca villa.
"You can understand someone shi**ing themselves, figur- atively-speaking, when police find a gun in your back garden, but this was literally," a source said.
"Gilligan going to the toilet was an urgent necessity.
"It wasn't a good day for him, but it was a sign for the police that this weapon could be hiding a lot of secrets and give detectives a great deal of useful information.
"Gilligan was present throughout the whole search, from the moment officers went over his garden with a metal detector to when it began to bleep and the gun was pulled out of the ground."
The revelation will add to the speculation that the gun found wrapped in plastic at Gilligan's four-bed home near the Costa Blanca town of Torrevieja could be the one used to kill Ms Guerin in a traffic light ambush on the outskirts of Dublin in June 1996.
Her brother, Jimmy, said it is "highly unlikely" it is the murder weapon, and the Herald revealed on Saturday that gardaí are "extremely sceptical" too.
However, Spanish police sources pointed to its rarity and the fact it was hidden "like a souvenir" in Gilligan's garden.
They said it was buried by someone who placed the gun in a hole and covered it with gravel and cloth used to stop weeds coming through before more gravel was piled on top.
One Spanish officer specialising in the fight against organised crime said he had seen only two guns of that model in the past 20 years.
Gilligan's embarrassing reaction to the find has led Spanish police to believe they may have made a significant discovery.
Gilligan was arrested last Tuesday along with five other people, including his British partner and his son, Darren.
The 68-year-old was acquitted in 2001 of Ms Guerin's murder but convicted of drugs trafficking and sentenced to 28 years in prison.
Sources said the raid on the his villa took place as he was preparing a delivery to Ireland of marijuana and prescription-only pills known as zimmos.
They said Gilligan was at home with part of the consignment and two other suspects were in a car outside with two boxes due to be sent through the post.
His British partner, Sharon, was the only woman arrested.
A Spanish National Police spokesman did not name Gilli- gan in a statement but said: "Investigators managed to intercept four postal deliveries in Spain in which 4kg of marijuana and 15,000 pills had been hidden.
"The well-known Irish criminal who allegedly headed the organisation was sentenced to 28 years in prison in 2001 and served 17 years.
"Irish investigators linked his organisation to the murder of the Irish journalist.
"The revolver that has been found is the same mark and model as the one used in the assassination of an Irish journalist in Dublin in 1996."
Gilligan has not yet been formally charged with any crime.
Investigations of the sort Gilligan now faces can take months and often years.
He can be held in custody on remand for up to four years without trial.
Ms Guerin was working for the Sunday Independent when she was shot dead in her car on the Naas dual carriageway on June 26, 1996.