Philip Seymour Hoffman, one of the leading actors of his generation, was found dead in his Manhattan apartment on February 2, 2014. File picture: Victoria Will

On the pretty streets of his neighbourhood in Greenwich Village, New York, Philip Seymour Hoffman would play the perfect family man, delighting locals with his easy-going charm, approachable air and the loving relationship he clearly had with his three young children.

They had given him a new sense of purpose in life and taught him not to be so obsessive about his work, the famously dedicated actor liked to tell interviewers.

In recent months, however, neighbours had noted a change for the worse in the Oscar-winner, widely regarded as one of the finest screen actors of his generation for his performances in films such as Capote, Doubt and The Master. One said he looked a "troubled soul".

People who ran into Hoffman at last month’s Sundance Film Festival in Utah had the same impression. He was there promoting his new movie, A Most Wanted Man. His usual scruffy appearance notwithstanding, he looked distinctly washed out and unwell.

He turned down a string of interviews, his spokesman telling reporters ‘he needed a minute and didn’t feel like coming down yet’. Hoffman later agreed to talk on the proviso there were no personal questions.

It was no secret he had struggled to stay free of the drugs he’d managed to give up 23 years ago — relapsing twice in the past two years.

But friends insist they were convinced he had it under control, seeking help every time he thought his addictions were threatening to overwhelm him.

It emerged on Monday that Hoffman had even helped other stars, including Batman star Heath Ledger — who was found dead in 2008 after an overdose of prescription drugs — to try to break free from heroin’s lethal appeal.

But clearly not everyone knew that his happy family unit had been broken and that he had recently become estranged from his children’s mother, costume designer and theatre director Mimi O’Donnell.

They had been a couple since working on a play together in 1999, but Hoffman had quietly moved out of the family home around Christmas and was renting an apartment nearby.

On Sunday morning, the 46-year-old actor failed to pick up their three children — Cooper, ten, Tallulah, seven, and five-year-old Willa — from a nearby playground, so Ms O’Donnell phoned Hoffman’s friend, screenwriter David Katz.

He in turn phoned Hoffman’s British personal assistant, Bella Wing-Davey, who unlocked the door to Hoffman’s £6,000-a-month flat and called emergency services.

Ms O’Donnell was distraught when she heard the news. Bundling her children into her car, she drove to Hoffman’s flat, leaving them in the car outside with its engine running, as she dashed inside shouting: ‘I have to see him!’

Officers reportedly didn’t allow her into the bathroom where he had died.

Police said on Monday that Ms O’Donnell told them that Hoffman appeared to be high when she saw him on the day before he died. He still sounded as if he was on drugs when she talked to him again on the phone at around 8pm.

The autopsy and coroner’s report has yet to be released, but police are confident Hoffman died from a heroin overdose.

Inside the fourth-floor apartment, investigators found the actor lying unconscious with a syringe stuck in his left arm. He was wearing just a T-shirt and boxer shorts. He still had on his glasses, said the police, who found no signs of a struggle.

The flat contained compelling evidence of heavy drug-taking — there were no fewer than 49 full glassines, the paper envelopes used to store heroin, as well as 20 used needles. There were also eight empty envelopes, most of them stamped with the logos Ace of Spades or Ace of Hearts — dealers’ identifying marks showing the ‘brand’ of heroin.

In the kitchen, there were bottles of prescription pills to treat anxiety and high blood pressure.

Police did not find a suicide note, and are understood to believe the death was accidental.

Did Hoffman start to fall back into drug use before or after he split from his long-term girlfriend? Showbusiness sources have suggested it may have been the reason for their separation.

Hollywood insiders say Hoffman and O’Donnell had been a devoted but understated showbusiness couple. Intensely private, Hoffman, whose parents divorced when he was young, did not like to talk about his family.

In a rare tribute to O’Donnell, he collected a Bafta for his role as Truman Capote in 2006 and said: ‘I want to say I love her and she looks really hot tonight.’ A few weeks later, he won an Oscar for the same role and forgot to mention her.

On Monday, friends said Hoffman and O’Donnell had been discreet about their break-up.

"Whatever was going on, they kept it quiet," said Meg Gibson, a former colleague of Hoffman. "All I knew is that Phil was a devoted father. He saw the children every day."

Ms Gibson also rejected any suggestion that Hoffman might have been in a relationship with Bella Wing-Davey, saying: ‘There was nothing romantic between them. Phil was a mentor for Bella. She was learning her way in the business as a family friend.’

Ms O’Donnell and her children released a statement saying: ‘We are devastated by the loss of our beloved Phil and appreciate the outpouring of love and support we have received from everyone.’

For now, police are trying to establish what sort of heroin Hoffman was taking.

There is speculation that the actor, who started using the drug as a student, may have inadvertently poisoned himself with heroin laced with fentanyl, a powerful opiate given to ease pain in cancer patients.

This dangerous concoction has recently been found in packets stamped with the Ace of Spades logo, such as the ones discovered in Hoffman’s home.

Heroin contaminated with fentanyl has been blamed for the recent deaths of almost 100 heroin users in the U.S.

"We don’t yet know what killed Philip Seymour Hoffman, but we do know there is a batch of heroin going round which is laced with fentanyl," said Meghan Ralston of the Drug Police Alliance.

Its victims ‘knew how much to inject, they knew how to be safe,’ she said. ‘You don’t realise this is a drug that is 100 times more potent than morphine.’

There were also reports on Mondaythat Hoffman may have been videotaped buying drugs the night before he died. He was seen looking ‘very sweaty’ as he withdrew large amounts of money from a local cash machine, which he handed to two men carrying messenger bags.

To add to the picture of his chaotic state, a witness told on Monday how she had seen the actor in a bar in Atlanta, Georgia, last Thursday — drinking, smoking and repeatedly running to the lavatory to such an extent that she’d found it suspicious.

Police have talked to Hoffman’s assistant Bella Wing-Davey, a pretty 28-year-old brunette.

The Cambridge graduate and granddaughter of the late Anna Wing — matriarch Lou Beale in EastEnders — has known Hoffman for nearly 20 years. They met when Hoffman, then an unknown actor, appeared in a 1996 play directed by her father, Mark Wing-Davey.

Best known in Britain for playing two-headed alien Zaphod Beeblebrox in The Hitchhiker’s Guide To The Galaxy, Mark teaches drama at a New York college and has collaborated with Hoffman on several plays.

"I’m not talking to folks right now about Phil," he said from his home in Greenwich Village.

Friends of Hoffman had only just got over a hoax claim on the internet that he had died, which circulated just days before his actual death.

Yet a handful of them have admitted they weren’t surprised by the news of his death.

‘Sadly, after being sober for years & years, he relapsed & has been grappling with his addiction for last few years,’ actress Kristen Johnson, a friend of Hoffman from college days, wrote on Twitter.

"I wish I could say I was shocked, but all I feel is a mixture of terrible grief, sad resignation and a powerful rage." The actor had said that he managed to stay off drugs for 23 years, until 2012, when he checked himself into rehab for snorting cocaine and taking prescription drugs.

In May 2013, he relapsed again and spent ten days in rehab. Even worse, this time he admitted he had snorted heroin for a week. He credited a ‘great group of friends and family’ for helping him seek the treatment he needed to recover.

But Hoffman was honest enough to admit there was no miracle cure, and that the threat of a relapse continually hung over him. Drug taking wasn’t simply ‘just a phase’ in his life, he confessed.

Inevitably, living on his own deprived Hoffman of the constant companions who might have prevented his final relapse. Having a family, he had argued, encouraged him to look after himself.

"When you have kids, health becomes more of a question than it ever did before," he said.

"You are aware of the maintenance of your life, while before you weren’t so aware of that."

The double tragedy of Hoffman’s death, it emerged on Monday, was that he had tried hard to save fellow star Heath Ledger and other celebrities from heroin.

"Philip didn’t hesitate when Heath’s people asked for help getting him clean," a friend of Hoffman told an entertainment news website.

"Philip spent hours talking to Heath, counselling and helping him, persuading him to choose life over drugs.

"That’s what is so tragic about Phil’s death — he would spend all this time on others giving them reasons to stay away from drugs and to live, and yet when it came to himself it’s as if he was spent: there was nothing left." -Daily Mail