The brother of one of the men stabbed to death in a park terror attack says he has "repeated nightmares of him being murdered".
Joe Ritchie-Bennett, 39, was killed in Reading's Forbury Gardens in June, along with James Furlong, 36, and David Wails, 49.
Khairi Saadallah was jailed for the rest of his life over the murders.
Robert Ritchie believes his brother's death could have been prevented and wants answers from the authorities.
Mr Ritchie, who spoke to the BBC from the family's home in Philadelphia in the US, said he felt "helpless" and had suffered "countless sleepless nights" since his brother's death.
"He didn't deserve to go out like that," he said.
"[It's] just like this horrible nightmare that you're living and you're going to wake up to someone shaking you saying 'hey, get up'.
"I have repeated nightmares of him being murdered the way that he was murdered. It's unbearable."
During his sentencing hearing, London's Old Bailey heard Saadallah "executed" Mr Furlong, Mr Wails and Mr Ritchie-Bennett as an "act of religious jihad" on the afternoon of 20 June.
It emerged after the attack that Saadallah, 26, was known to MI5, had told the government about his earlier involvement with a banned Libyan militia and had a long history of committing violent crimes.
Saadallah had been told just before his release from prison that the Home Office wanted to deport him, but it was not legally possible due to the situation in Libya.
Mr Ritchie joined the calls made by the father of Mr Furlong, Gary, for the home secretary to "tell us why" the killer was deemed safe to be free.
Mr Ritchie added the family was "not looking to assign blame to anyone other than the terrorist".
He said: "However, if missteps are uncovered, it is our great hope and desire that they will be properly remedied in a positive outcome, in so that this never occurs to anyone or their families ever again."
The prime minister's spokesman previously said the government "always tries to remove foreign national offenders where possible".
In law, what are known as the Hardial Singh principles place certain limits on the government's power to detain people ahead of deportation
Mr Ritchie said his brother had "the absolute best smile in the world" which was "infectious".
"He could capture you from a mile away with that smile, he was so loving. He was so brilliant. He was so handsome," he said.