A probe launched in 2015 into the alleged attempted murder of the former Liberal leader's gay ex-lover Norman Scott was closed last year.
Gwent Police had thought Andrew Newton - the man allegedly hired to kill Mr Scott - was dead.
But the force has told BBC Four documentary The Jeremy Thorpe Scandal that new information has come to light, suggesting he may still be alive.
Reacting to the news, Mr Scott, 78, told the programme: "I just don't think anyone's tried hard enough to look for him. I really don't.
"There must be people who knew him and there would surely be a record of him dying.
"I thought (Gwent Police) were doing something at last and soon found out that absolutely they weren't, they were continuing the cover up as far as I can see".
The documentary, which is due to be broadcast at 10pm on Sunday, investigates the alleged plot to murder Mr Scott, who was involved in a relationship with Mr Thorpe in the early 1960s, when homosexuality was illegal.
Mr Thorpe, who died in 2014, was acquitted of conspiracy to murder after an Old Bailey trial in 1979.
A fresh probe was launched by Gwent Police in 2015 after new claims emerged.
But Mr Scott was told the investigation was over after the force concluded Mr Newton, who was jailed for firearms offences over the shooting of Mr Scott's dog on Exmoor in 1975, was no longer alive.
Gwent Police told documentary makers: "Enquiries were completed which indicated Mr Newton was deceased.
"We have now revisited these enquiries and have identified information, which indicates that Mr Newton may still be alive.
"As a result, further enquiries will be conducted to trace Mr Newton to assess if he is able to assist the investigation."