MURDER SECRET BEHIND DOWNEY’S DRUG ARREST

ROBERT DOWNEY Jr. was so desperate for drugs that he risked his life by running around with a junkie who’s the prime suspect in the bloody murder and dismemberment of a 20-year-old girl.

The “Ally McBeal” star got his drugs from Albert Aleixo, who partied with Downey in a motel room in the hours before the star’s April 24 drug bust, charge insiders close to the case.

Now police want to talk to Downey to find out what he knows about the grisly murder of the young woman — who was found cut in half!

The ENQUIRER investigated Downey’s recent bust and discovered startling details of his insanely reckless life — and got an exclusive eyewitness account of his wild encounter with police.

“Downey doesn’t realize how lucky he is to still be alive,” a police source told The ENQUIRER. “If he’d said one wrong word to this unstable guy (Aleixo) when they were high, anything could have happened.”

Downey was busted outside a Los Angeles motel along with 45-year-old Aleixo, who is already awaiting trial for possession of heroin and intent to sell narcotics.

“Aleixo is a known associate of the Russian Mafia,” charged the police source. “He is the main suspect in the murder of his girlfriend, 20-year-old Russian prostitute Alissa Sologubova. Police want to know if Aleixo ever confessed his girlfriend’s murder to Downey.”

The murder was incredibly grisly — revealing the sicko killer’s capacity for horrific violence.

“In February last year, Aleixo reported Alissa missing,” disclosed the source.

“She had been murdered, her body cut in half and dumped in a sewer. The top half of her body washed down to the Hyperion Treatment Plant in El Segundo, where it was found by a worker.

“The bottom half was never found. Aleixo immediately became the prime suspect.”

Now chilling new questions are exploding to the surface: How much does Downey know — and will someone try to silence him?

After the bust, the star was hospitalized and is being closely monitored. “Right now being under 24-hour watch is the best thing for Robert,” disclosed a Hollywood insider.

“He’ll be able to get clean once again — and stay free from the clutches of the Russian mob.”

Aleixo, too, is facing a day of reckoning. He is awaiting trial on two prior felony arrests — drug possession and possession with intent to sell.

“The cops found 11 eight balls of cocaine (about 1.3 ounces) and decided this quantity justified an intent to sell charge, but Albert has a $150-a-day habit so it makes perfect sense for someone like him to have that amount for personal use,” his lawyer Peter L. Knecht told The ENQUIRER.

“The second charge involves just five grams of heroin, which for a drug addict like Albert is a negligible amount.”

Downey’s bizarre life started unraveling again when he took an $85 cab ride from his Malibu home to L.A.’s Baldwin Motel where he rented a $45 room with Aleixo, say sources.

A short while later, says local resident Kent Hawkins, “I witnessed Downey’s pal, who police later called Aleixo, hand cash to a dealer for five small packets of what appeared to be cocaine. Around here rock cocaine sells for $50 a pack.

“Downey said to the dealer, ‘If it’s good, we’ll be back for more.’ “

Around midnight, Hawkins says he saw Downey emerge from the motel — disoriented, disheveled and obviously high. “A patrol car passed by. The cops got one look at Downey and stopped to question him.

” ‘Is this your room? Can I take a look inside?’ asked a female officer.

“Downey told her: ‘It’s not my room. I’m just visiting a friend.’

“He began running and I saw him drop one small packet in front of Room 108 and the second by a trash can.

“Then he made a beeline across the street into an alley where he tried to squeeze between a brick wall and a chain-link fence. But he couldn’t fit — and the officer finally nabbed him.

“While this was going on, Aleixo appeared from the room, pulling up his pants. A male officer called out to him, but Aleixo took off. The male officer nabbed him, and both Downey and Aleixo were handcuffed.

“Downey said, ‘I’m just high, you don’t have to take me to jail. I’m not doing anything wrong. If you arrest me, I’ll lose my job.’

“When the officer then asked if they were having sex, Aleixo quickly replied, ‘Yes, but I wasn’t doing the drugs. It was Robert.’ Both were put in the patrol car and taken off to jail.”

Aleixo’s attorney Knecht denies his client is a member of the Russian Mafia and told The ENQUIRER, “Neither Albert nor Robert is gay.”

Aleixo was released on his own recognizance. Downey was released into the custody of his parole officer, and then admitted into a detox center at Cedars-Sinai Medical Center.

Distraught, the fallen star began chain-smoking and making tearful phone calls to pals as an uncertain future, clouded by a murder investigation, loomed ahead.

— JOHN SOUTH, RICK EGUSQUIZA and ROBERT BLACKMON