Stephen Merchant admits he found it "utterly shocking" how the British police handled the serial killer investigation depicted in BBC drama Four Lives.
The actor, 47, portrays sex predator Stephen Port, who drugged and raped four men before dumping their bodies near his home in Barking, east London.
Port, known as the Grindr Killer, used the gay dating app to target young men.
The three-part factual drama - which also stars Sheridan Smith - is told from the point of view of the victims' families, focusing on their fight to uncover the truth about what happened to their sons and brothers.
In December, inquest jurors found the shambolic inquiries probably contributed to the deaths of three of Port’s victims - Gabriel Kovari, Daniel Whitworth and Jack Taylor.
Loved ones of the three men and those of the first victim, Anthony Walgate, raised concerns that something more sinister had happened after each of the deaths but were ignored by investigators.
Merchant, known for Extras, The Office and movie Fighting With My Family, said: "The idea that in 2014 and 2015, you’re still coming up against silence, you still can’t get through the door when you’re trying to help.
"You’re not even trying to obstruct, you’re trying to help. You’re saying, 'Let me ask questions. Let me see the forged letter that was found on one of the victims,’ and the police go, ‘We don’t need your help.’
"I mean, that is what I found so utterly shocking… I felt it was an important piece of work."
Port was found guilty of four murders following a trial at the Old Bailey and handed a whole life sentence in November 2016.
The police watchdog is looking at whether any charges of gross misconduct should be brought against officers involved in the initial inquiry which allowed Port to slip through the net.
Merchant added: "That’s the thing that seemed most devastating and heart-breaking, you think this is something that’s going to be set in the 1970s, with the reactions of the police and the sort of hum of homophobia that’s through it all, and instead, 2014.
"It’s shocking."
Four Lives launches at 9pm on January 3 on BBC One.
