Heroin and crack were killing Whitney Houston’s troubled daughter before she was found unconscious in a bathtub, The National ENQUIRER can reveal in a bombshell world exclusive.

In an explosive tell-all, a man who admitted to using those deadly drugs with Bobbi Kristina Brown said she was on “a tragic downward spiral” at the time of her horrific Jan. 31 medical emergency.

The informant spoke out as the 21-year-old was still clinging to life at Atlanta’s Emory University Hospital, surrounded by family and friends including her devastated father, Bobby Brown.

“Whether heroin or crack – or other drugs – played a role in what happened to her will be determined. But I know how bad off she was in the months before this happened,” said the man, whose name is being withheld by The ENQUIRER.

The snitch – who’d gone to school with Bobbi Kristina and has remained close to her – claims her drug use escalated in the aftermath of Whitney’s death in 2012.

“She told me her mother’s death ‘gutted her.’ Those were her exact words,” the man said.

The ENQUIRER has past reported that Krissi – as she’s known to friends – began abusing drugs at age 17. We’ve also published shocking photos of her snorting cocaine and taking hits of marijuana from a bong.

But her lethal turn to heroin and especially to crack – the addictive street drug that turned Whitney into a junkie – has never been disclosed until now.

At the time of the tragedy, Bobbi Kristina was spending upwards of $5,000 a week on drugs, according to the snitch.

He said he came forward to help piece together the events that led up to Bobbi Kristina being found unresponsive in a water-filled bathtub at the Roswell, Ga., home she shared with her partner Nick Gordon.

One clue that could help authorities investigating the incident would be to locate “the secret vault-like chamber” hidden in her home, he said.

“That’s where Krissi keeps her drug stash and cash, and also where Nick keeps his guns,” the man told The ENQUIRER.

He said other answers could come from two local drug dealers who supplied Bobbi Kristina. “These two are known as ‘Steve-O’ and ‘G,’” he said. “They would either come to her place or she’d meet them in her car at a designated location.”

A lawyer for her father told The ENQUIRER it was categorically false that Bobbi Kristina had used either heroin or crack.

But The ENQUIRER source confirmed that he had used drugs multiple times with Bobbi Kristina.

He said: “Before Whitney died, Krissi was hooked on coke and marijuana, and also had a problem with Xanax.” According to the source, Bobbi Kristina’s descent into harder drugs unfolded in the months after her mother passed.

In an eerie foretelling of Bobbi Kristina’s tragedy, Whitney was found dead in a water-filled bathtub at a Beverly Hills hotel on Feb. 11, 2012. Cocaine and other drugs were found in her body.

After that, Bobbi Kristina first turned to Roxicodone, a synthetic drug chemically similar to heroin known as “Roxys,” said the man. “She really liked the feeling she got from Roxys. She snorted them, and she’d ask me, ‘Can you get them for me?’”

“Not long after that she traded up to ‘full-blown’ heroin. We’d use heroin together at friends’ apartments, at parties.”

The man also said he was aware of a specific incident last September when his close pal, Bobbi Kristina and Nick all smoked crack together at a friend’s birthday celebration in a Roswell park.

As the party was winding down, the three hopped into Bobbi Kristina’s car near the park “and lit up a crack pipe,” he said.

“Nick’s just as bad,” said the man. “They’ve been in this self-destructive cycle for a long, long time.”

The man said Bobbi Kristina and Nick recently began sleeping in separate bedrooms because of continued fighting in their relationship. Police were called to their Roswell residence on Jan. 23 over a reported fight. But when cops arrived, no one was home.

The man said: “The Wednesday before [Krissi was found], she texted a friend of mine, saying, ‘I don’t think things between me and Nick are working out.’

“Krissi wanted to leave him, and had plans to move to California to follow in her mother’s footsteps as a singer. Sadly, it looks like her dreams will never be fulfilled.”