Police announced a slew of new murder charges in connection with a "horrific" slaying which left six people -- including two innocent bystanders -- dead in a Surrey, B.C., apartment 18 months ago.

Four alleged gangsters now face murder charges in the Oct. 19, 2007, killings, which shocked the Lower Mainland and kicked off an increasingly violent string of killings in the Metro Vancouver area.

"It was a gangland execution fuelled by drugs, gangs and guns," said RCMP assistant commissioner Peter German on Saturday.

James Kyle Bacon, who is linked to the notorious Red Scorpions gang, now faces a first-degree murder charge in the death of Corey Lal, a 26-year-old who had criminal ties, police said.

Matthew James Johnston and Cody Ray Haevischer also face new murder charges for their parts in the deaths, police allege.

Johnston, Haevischer and Bacon were all arrested on Friday -- the same day as their associate Dennis Karbovanec pleaded guilty to three counts of second-degree murder.

Karbovanec admitted to murdering Chris Mohan, a 22-year-old innocent bystander who lived in the building where the slayings took place and was accidentally caught in the crossfire.

Karbovanec's other victims were Ryan Bartolomeo, 19, and Michael Lal, 26 -- two young men police say were linked to the criminal underground.

The other innocent victim, Ed Schellenberg, was a 55-year-old father of two who maintained the tower's fireplace system.

Edward Narong, 22, who had a criminal background, was also slain.

"These arrests did not just happen," German said. "They were the culmination of intense investigative work performed by literally hundreds of investigators."

Since the slayings, there have also been 100 shootings in the regions -- many of them linked to gang activity.

"We've had to prioritize other investigations in order to break the back of this file, which we always viewed as critical to the present gang landscape," German said.

"It is only through solving these serious murder cases that we can take wrongdoers off the street for a prolonged period of time."

Eileen Mohan, speaking at the news conference, said officers were always supportive in the aftermath of her son's death.

"I wish I was not standing here and my life was the usual life that I knew prior to Oct. 19, and my son would be beside me. But I know he's smiling from heaven today."