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.

The arrests were bittersweet for the family, who authorities speculate were wrongly targeted by the gunmen as rival gang members.

“I’m happy for the community but it does nothing for me, personally,” the boy’s grandfather William Shannon told The Associated Press on Friday. “It won’t bring my grandson back and there will always be a void in the family.”

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

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).”

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 shot in the left leg, and the boy’s 55-year-old grandfather was hit in the wrist. Neither of their injuries was considered serious.

William Shannon said Aaron attended school in Compton and spent weekdays at his grandfather’s home there. He went back to his father’s Los Angeles home only on weekends.

The grandfather said he had planned to celebrate his 56th birthday next Monday with his grandson.

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

William Shannon said he didn’t know the motive for the attack. The two men walked down the alley past the home and out of view without saying a word but then apparently returned, he said, and one began firing through the fence and some bushes.

The grandfather said he heard four gunshots and his first thought was: “Wow, I hope those guys were all right.”

Then the gunman stepped back away from the bushes and William Shannon saw the gunman fire four more shots from less than a dozen feet away.

“I could see him face-to-face,” he said.

He was struck and ran inside, where he found his grandson lying on his back in a pool of blood, still in his costume. He said the boy’s father, Aaron Sr., 25, had brought the boy inside.

The bullet had struck him in the back of the head and exited above his right temple, the grandfather said.

Aaron Jr. was declared brain dead Monday. William Shannon said he and his son decided to donate the boy’s kidneys and pancreas.

“He’ll live in someone else,” he said. “We may not know him but he’s out there somewhere, here on earth and in heaven.”

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.