UPDATED 12/30/10 6:08 p.m.

CHICAGO (CBS) — A teenager wanted to steal a collection of guns so he could sell them for quick cash. That’s why police say Bernard Williams, 18, shot and killed a Chicago Police officer last month.

Chicago Police Supt. Jody Weis formally announced the charges Thursday morning at Police Headquarters, 3510 S. Michigan Ave., in the arrest in the murder of SWAT Team Officer David Blake, 45, on Nov. 22.

Williams, of the 2500 block of West 81st Place, was charged with Blake’s murder around 8 p.m. Wednesday. Williams is charged with one count of first-degree murder, and one count of armed robbery with a firearm. He was ordered held without bond Thursday.

Investigators say Williams, a member of the Gangster Disciples, has since confessed to the crime.

Blake was found shot to death inside his sport-utility vehicle last month. His vehicle was parked in the 2900 block of West Seipp Street, a block that terminates at a dead-end in the Wrightwood neighborhood.

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Prosecutors say Blake was dating Williams’ sister. They say Williams made up a story to get the off-duty officer alone, to kill him and steal the keys to his house.

All so he could take Blake’s extensive gun collection, which he had seen at the officer’s home when he was invited there to watch a football game back in October.

“While there, the defendant’s sister showed the defendant around the victim’s house. It was then that he saw a safe that was located in one of the bedrooms,” said Cook County Assistant State’s Attorney Jamie Santini.

Prosecutors say Bernard Williams asked Officer David Blake to show him the guns that were kept in the safe. Blake did.

“The defendant at that point saw that the victim had kept several guns in that safe,” said Santini.

“Williams’ motive was to shoot and kill Officer Blake for the purpose of stealing weapons inside of his residence,” said Weis.

Prosecutors say on November 22, after Williams was at Blake’s home, he called his sister saying he had left something at Blake’s house.

The story was a ruse to get Blake alone with Williams. Blake picked Williams up.

Before going to his home though, Williams told him to drive to a secluded spot on Seipp Street, because he needed to get something from a friend’s house.

“The area the defendant directed the victim is referred to as “Dead Man’s Alley” because it is a remote location,” said Santini.

Once there, prosecutors say Williams went to a house and pretended to meet a friend, then returned to Blake’s SUV.

“The defendant produced a gun and shot the victim six times, killing him,” said Santini. “The defendant then took the victim’s car keys, which also included keys to the victim’s house and to the safe in which the guns were kept.”

And then, within an hour of the shooting, he went to Blake’s home to steal the guns, Weis said. But he couldn’t get in because police had already arrived.

“We were fearful because of his [Blake’s] weapons that the intent may have been to rob him and get those weapons, so we put officers on his apartment and his mom’s apartment right away to prevent that from happening,” said Weis. “And we’re pretty confident that we were able to do that.”

Williams likely chose the isolated block of Seipp Street “possibly not to be discovered or not to be observed by anyone else,” Wentworth Area Detective Sgt. Shawn McGavock said at the Thursday morning news conference.

Prosecutors say Williams is homeless and a high school dropout, and he wanted to sell the guns for cash.

Blake’s gun and wallet were found at the scene, which originally led police to believe robbery was not a motive. A cigarette was still in the mouth of the 45-year old police officer when his body was found.

Suspect Williams reportedly told friends interviewed by police he had shot Blake and confessed to the crime late Wednesday. The friends were brought before a Cook County grand jury on Wednesday, the source said.

“There is satisfaction in having obtained charges, yet there is great sorrow for the loss; the senseless loss in human life, and for the evil which exists in the hearts of some men,” Weis said.

Police say they were able to zero in on Bernard Williams as a person of interest after completing what the superintendent calls a thorough and unrelenting investigation.

“They reviewed thousands and thousands of computer files and phone records to develop leads,” said Weis.

After the press conference, Deputy Supt. Steve Peterson said that Williams knew Blake’s line of work when he allegedly shot him.

“He knew he was a Chicago Police officer,” Peterson said.

He praised the work of the Wentworth Area detectives and gang intelligence investigators.

“They did a great job, they were the ones who painstakingly went through all the evidence,” Peterson said.

Wearing a white paper jumpsuit in court Thursday, Williams said nothing and showed no expression as prosecutors laid out the case, but he appeared to be listening intently.

Williams’ family attended the bond hearing but left without commenting.

Williams’ family spokesperson, Apostle Columbus Bland, a member of the Glory to Glory Family Christian Center, said Williams’ mother attended the church for more than 18 years.

“On behalf of Bernard, the only thing that I can say is that we will be praying for him,” said Bland. “We’re not here to defend him, or convict him.”

He said Williams’ mother is grieving, “as any mother would.”

“As far as I know, he was a good boy,” said Bland. “As far as I know, but you don’t know when you don’t go home with them.”

No one answered the door at Williams’ home Thursday morning.

Neighbor Veronica Rogers, 41, said she believed Williams attended Bogan High School and was into wrestling. Williams’ assistant public defender said in court Thursday he did not graduate from high school.

Another neighbor who said she usually presides over the 81st Street Block Club’s monthly meetings said the family and Williams “kept a low profile. As far as I know he’d never been in any trouble.’’

Williams has one prior arrest on a misdemeanor charge, McGavock said.

He will appear for a preliminary hearing Jan. 18.

Officer Blake was a decorated police veteran who had been on the force for 15 years and had received 84 awards for distinguished service. He was also a member of the Chicago Police Department Enforcers football team.

CBS 2’s Mike Puccinelli, Suzanne Le Mignot, Newsradio 780’s Bernie Tafoya, and the Sun-Times Media Wire contributed to this report.