(CNN) -- Joe Saenz rubbed his hands together and smiled as he walked up to a Los Angeles County, California, home.

His demeanor gave no clues as to what would happen next.

Once in the front yard of the home, police said, he drew a gun and shot the victim several times, then in the head, execution-style. The slaying, caught on tape in October 2008, was over drug money, police said.

Los Angeles County Sheriff's Department Detective Traci Gonzales knew there were drug and gang ties to the slaying, so she showed the videotape to Los Angeles Police Department Detective Ron Chavarria, a veteran on the gang beat.

At the time, Gonzales had no idea that videotape would break open a case that has frustrated local and federal authorities for a decade.

Saenz had eluded authorities for so long, they thought he might be dead. Now, they knew Saenz, wanted in connection with four murders, a kidnapping and a rape, was still out there.

Saenz is the newest person on the FBI's Most Wanted List, after three fugitives on the list were recently caught.

A member of the Cuatro Flats gang, Saenz first stepped into police crosshairs in 1998. He was wanted for shooting and killing two rival gang members at a housing complex in the Hollenbeck area of Los Angeles.

Then, he was a fresh-faced 22-year-old with a dark side.

"Just gruesome, gruesome, cold-blooded," Chavarria said. "Just walks right up to these guys, shoots them and then just walks over and stands over them and shoots them each in the head and casually walks away."

They knew he was dangerous, and they had to find him.

"We had pictures of him constantly on our dashboard so that there was no question if we saw this guy we knew who he was," Chavarria said.

Within days, Saenz -- also known as Jose Saenz, Louie Louie and more than two dozen other aliases -- was wanted for a third murder.

Investigators said Saenz suspected the mother of his child, Sigreta Hernandez, would turn him in, so he kidnapped, raped and murdered her at his grandmother's house.

"He leaves a note indicating that his grandmother has nothing to do with the murder and that he left the scene. And from that point on he became a fugitive for the next 10 years," Gonzales said.

FBI Special Agent Scott Garriola believes Saenz graduated from local gangs to international drug trafficking -- crossing the border with falsified documents.

"He's believed to be linked with the cartels in Mexico, bringing large amounts of narcotics into the Los Angeles area, Garriola said. "And I think the reason for the last crime was a direct relationship to the cartels."

He's eluded police for years, and has reportedly made statements that he plans to kill a police officer if they ever find him and arrest him, according to the FBI's Most Wanted Web site.

Now, authorities are hoping the video from October 2008 will help them get one step closer to the man who they say terrifies those who know him just as much as his victims.

"We heard rumors that not only citizens were afraid of him, but also his own gang members were afraid of him," Chavarria said.

The chilling video, Chavarria said, shows why Saenz is so dangerous.

"It looks like he really enjoys what he does, he enjoys killing people," he said.