Time and time again, Officer David Blake tried to help out his girlfriend's little brother, a diminutive high school dropout with a troubled past and, reportedly, gang ties, authorities said.

Upon visiting Blake's home on the Southwest Side to watch football in October, Bernard Williams was given the grand tour, peeking through bedrooms and closets. Blake, a 45-year-old SWAT officer, was even comfortable enough to let Williams, 18, see his gun collection, which was kept inside a bedroom safe, authorities said.

On the last day of his life, Blake was there for Williams, agreeing to pick up the young man, who passed a message through his sister that he had left something behind at Blake's home on one of his visits, authorities said.

But Cook County prosecutors said the message was actually a ruse by Williams, who had plotted to kill Blake and steal his weapons cache. The plan, they said, included luring Blake to a secluded area, killing him, then stealing keys to the officer's house and safe.

Williams would then clean out the safe and sell the weapons on the street, prosecutors said.

Blake was fatally shot on Nov. 22 in his sport utility vehicle on an unpaved alley on the Southwest Side known as "Dead Man's Alley."

But Williams, who reputedly is a member of the Gangster Disciples street gang, never got the chance to steal the guns, because when he arrived at Blake's home after the shooting, police were already there, authorities said.

"We were fearful, because of his weapons, that the intent may have been to rob (Blake) and get those weapons," police Superintendent Jody Weis said Thursday in announcing murder and armed robbery charges against Williams. "We put officers on his apartment and his mom's apartment right away to prevent that from happening, and we're pretty confident that we were able to do that."

In court, Williams, who wore a light-colored jumpsuit and bright turquoise gym shoes, spoke only to identify himself to Judge James Brown.

Prosecutors told the judge that Williams began formulating a plan to steal Blake's weapons soon after seeing them.

On the night of the shooting, Blake picked up Williams and drove him to an alley in the 2900 block of West Seipp Street, after the teen said he needed to pick up an item from a friend, Assistant State's Attorney Jamie Santini said. After getting out of Blake's SUV, Williams got back in moments later with a handgun and shot the 15-year veteran six times, killing him, Santini said.

At a news conference Thursday, Weis said detectives had to restart their investigation when early leads yielded nothing. But weeks of poring over Blake's computer and phone records and interviews led investigators to Williams, who was arrested Tuesday.

Williams, authorities said, admitted to shooting Blake and masterminding the robbery. He also told several others that he killed Blake, Santini said.

Friends and acquaintances described Williams as homeless, moving between friends and relatives for a place to sleep.

Williams was arrested in March on an assault charge, but the case was dropped when the victim failed to show for court.

Angela Starnes, Williams' aunt, said that she was shocked and "disgusted" when she received the news about his arrest.

Starnes, who lives in the Downstate city of Danville, said Williams stayed out of trouble growing up and regularly went to church with his mother. But things started to change recently, as he attended school sporadically and began hanging out with gangs, she said.

"Once he turned 17, he started to turn," she said. "Instead of staying on that positive path that my sister had him on. I don't know what happened. He was raised very well."

Tribune reporter Dawn Rhodes contributed to this report.

wlee@tribune.com