LOS ANGELES -- Two reputed gang members were arrested in the Halloween killing of a 5-year-old boy who was shot in the head as he showed off his Spider-Man costume in his backyard, police said Friday.

Marcus Denson, 18, was arrested Thursday at a home near the killing site after a brief chase, Los Angeles police Capt. Dennis Kato said.

Leonard Hall Jr., 21, was arrested at about 2 a.m. Friday at an apartment building several miles north of the site, Kato said.

The men, both reputed members of a local street gang, were booked on suspicion of murder and remained jailed Friday, he said. Their bail was set at $1 million each, according to the Los Angeles County Sheriff's Department.

"We knew, as a law enforcement agency, that eventually we were going to get these guys," Kato said. "The only thing that saddens me is we failed to protect him (the boy)."

Aaron Shannon Jr. was flexing the muscles of his Spider-Man costume and posing for photographs in the backyard with his uncle, grandfather and cousin Sunday afternoon when he was shot, police and relatives have said.

Investigators believe Hall and Denson were in an alley behind the house when Hall pulled a handgun and fired several shots through a chain-link fence into the yard, Kato said.

The boy, who was getting ready to go to a party, was shot in the back of the head and died at a hospital the next day.

His uncle, Terrence Shannon, 27, was hit in the wrist, and his grandfather William Shannon, 56, was hit in the left leg. Neither of their injuries was considered serious.

The home is located in disputed gang territory and the attackers may have mistakenly believed that members of a rival gang lived there, police have said.

Investigators from the LAPD, FBI and Sheriff's Department suspected gang involvement and were able to identify suspects based on witness tips spurred by a $75,000 reward. They also were aided by video recorded by a nearby business, Kato said.

The grainy tape showed two men running across the street after the shooting. It couldn't be enhanced enough to identify faces but it provided a general description of the suspects' clothing and hairstyles, Kato said.

The gun believed to be used in the attack was not found, he said.