SEARCH: Forensics examine an area near where Philip Strickland was gunned down
Seales was sentenced to life with a minimum 14-year tariff while Ian, who gave evidence against his dad, was jailed for just four years.
Prison sources say they cannot understand why Jason Weir took his own life when he was due to be freed later this year.
"His death has come as a huge shock to everyone, staff and inmates alike," explained a Magilligan insider.
"Jason was popular enough with the other prisoners, he didn't have any enemies and was always talking about getting out soon.
"He seemed to have withdrawn into himself these last few weeks and isolated himself from his friends, but no one had any idea that he was feeling suicidal.
"The inmates and staff feel very low at the moment. Regardless of what crime a prisoner is serving a sentence for, they don't deserve to have their lives ended so suddenly."
Our source said that Jason Weir's jailed father Jimmy Seales has taken his son's death badly. Unlike his other son Ian, he was on good terms with Jason.
A spokesman for the Prison Service refused to comment on the Magilligan suicide, saying it never talks about individual inmates.
The Weir family is having a private funeral ceremony for Jason, and has not placed death notices in any newspapers.
His victim Philip Strickland was killed with a shotgun nine years ago after being abducted by Jason and Ian Weir at their father Jimmy Seales' yard.
Seales blasted the 36-year-old in the leg before bundling him into the boot of a car and driving off. Despite being severely injured Strickland tried to flee, but was shot again in the face causing catastrophic injuries from which he died.
Also convicted of his murder was Stephen McCaughey (33), who was sentenced to life with a minimum 10 years.
Seales is currently appealing against his murder conviction, telling Sunday Life he was physically incapable of firing the fatal shots because of horrific arm injuries he suffered in a beating the year before.
Ian Weir was freed in 2019 having served his minimum four-and-a-half year term for murder, while McCaughey is due for release next year.
During Seales' trial it was claimed he had offered his sons Ian and Jason £500,000 each to take the rap for the Strickland killing and put him in the clear. He has since denied this in behind bars interviews with this newspaper.
