Almost two decades after the 37-year-old was shot dead at a suburban motel his family continues to live a "nightmare" over the slaying.

"I have not had a decent day for 19 years thinking about what could have happened to my brother," sibling Marc told reporters on Thursday when police launched another appeal for information to solve the crime.

"He left a heartbroken mother, a brother and a family. He didn't deserve what he got. He was a good family member ...he was a thug to a certain degree."

Detectives have released new crime scene photos which they say could be key to solving the mystery. They also revealed Mladenich was shot with a 0.25 calibre self-loading pistol - small enough to fit in the palm of a hand.

Mladenich, who was crime boss Carl Williams' bodyguard, was killed in front of three people in a room at St Kilda's Esquire Motel on May 16, 2000.

The motel was well-known for drugs and prostitution, but the motive for the killing remains unknown, Detective Acting Sergeant Jesse Maxwell said.

"There is still someone in our community, who after 19 years, who committed an extremely violent and life-ending act and that person needs to be held to account," he said.

"We believe this is solvable and we intend on solving it."

Det Sgt Maxwell said there were people still alive who knew why Mladenich was killed and by whom, and several were involved in the organisation of the slaying, and the aftermath.

"Richard deserves his justice. Richard never deserved to die in the way he did," he said.

It is believed the shooter entered the motel room through an unlocked door.

"Over 19 years have gone, people's allegiances and loyalties change," Det Sgt Maxwell said.

"A lot of people who were big players back then are not around. So perhaps the fear of talking has passed. Maybe they've seen the light and (will) come forward."

Mr Mladenich was childhood friends with Williams' wife, Roberta.

Marc said the family was unaware of his gangland associations.

"We want for the coward who did this to be put away for a long time," he said.

"He deserves to be incarcerated like my brother has been. I want justice for my brother and my family."

Victoria Police have previously been criticised for offering a $1 million reward to solve the gangland killing, given Mladenich's criminal links. That reward still stands.

Australian Associated Press