walked Elizabeth Cullen, 63, from the shallow to deep end

They also suggested that he may recently have been

The former U.S. Army Chief of Staff's nephew accused of drowning his mother in the pool of their $1.9million home had been taking heroin since high school and had recently been in a treatment facility.

Sources close to the family told Daily Mail Online that Dennis Cullen, who was known as 'Dyson', he had a history of drug problems before he allegedly dragged 63-year-old mother-of-five Elizabeth Cullen to the back of their Cold Spring Harbor, Long Island, home and held her under the water until she died.

Mrs Cullen and her wealthy investment banker husband Dennis were members of Cold Spring Harbor Social Club but withdrew a lot from the local social scene when their son's drug problems began

Cullen, 23, was arrested and charged with her brutal death after telling police he was surprised his mother struggled so much when he dragged her from the shower, walked her through the house and then pulled her from the shallow to the deep end of their pool in a headlock.

Police have said the former varsity lacrosse player at Cold Springs High School flipped when she 'poked' him during a row over his anti-psychotic medication.

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Denis Cullen Jr (left) has been charged with murder after his mother, Elizabeth Cullen (right), was found dead in the pool in their backyard. The 63-year-old is pictured in her College of the Sacred Heart yearbook photo in 1975

He is pictured walking to a police van before his arraignment on murder charges in Central Islip, New York

Cullen, who has three sisters, is believed to have taken a train to New York after his mother's death to confess to his sister. When he returned, he was arrested by police already at his home.

Prosecutors say Cullen made a 'chilling' confession to detectives, but pleaded not guilty to second-degree murder and was held on $8million bail.

Daily Mail Online has exclusively obtained Mrs Cullen's yearbook photo from her when she was a student in 1975 at The College of Sacred Heart in Newton, Massachusetts, which has since merged with Boston College.

Inside Edition have also obtained photos of the family in happier times. They revealed a picture of Mrs Cullen enjoying a holiday on Italy's Amalfi Coast.

Mrs Cullen, whose nickname was 'Wink', the friend of 20 years said, was a devoted mother and sweet, kind person.

Nick Schenier, who lives near the Cullen family, alluded to mental illness being involved when he spoke to Daily Mail Online and said that Mrs Cullen was one of the nicest ladies you could ever meet.

Cullen is seen with family and friends enjoying happier times. Police have said he had a history of mental problems and was taking anti-psychosis medication

Mrs Cullen is seen enjoying a holiday on Italy's Amalfi Coast. She was a member of Cold Harbor Social Club - but became distant when her son's drug problems started deteriorating

Cullen's grandfather was Major Gen. George William Casey (left), the highest ranking officer to die in the Vietnam War. His uncle is the former Army Chief Of Staff, George Casey Jr (right)

A judge held him on an $8million bail during his first court appearance on Thursday morning.

Cullen's grandfather was Maj. Gen. George William Casey, the highest ranking officer to die in the Vietnam War. His uncle is the former Army Chief Of Staff, George Casey Jr.

Cullen's father is a Managing Director at RBC Capital Markets and married Elizabeth in 1983. The couple had a total of four children

Lloyd Harbor Police carried out a 'welfare check' at the family home at about 4pm on Wednesday when they made the grim discovery.

It is unclear what prompted the police visit but Cullen Jr. wasn't at the property when they arrived.

He returned to the house while the police were investigating and was immediately arrested without a struggle.

The 23-year-old was led in silence to a police van on Thursday morning and was driven to court for his first appearance in front of a judge, News 12 reported.

He is not believed to have a prior criminal record and pleaded not guilty to one count of murder in the second degree.

Police would not confirm reports that he called his sister to confess to the murder or that he had a history of mental illness.

Officers cordoned off the road outside the home and Suffolk County Homicide Squad is now investigating.

Cullen's father is a Managing Director at RBC Capital Markets and married Elizabeth in 1983. The couple had a total of four children.

Their daughter, Courtenay Cullen, is in the Army.

George Casey Jr is pictured with Hulk Hogan's ex-wife Linda, his son Nick Hogan and daughter Brooke Hogan

Mrs Cullen was found in the swimming pool (circled) in the backyard of her home

The Cullen family lives in this house in the affluent Cold Spring Harbor area of Long Island, New York

The sprawling Long Island estate was surrounded by unmarked police vehicles as Cullen made his first appearance in court

Police cordoned off the area surrounding the swimming pool as they begun the investigation into the brutal killing

Police surrounded the property, but said none of the Cullen family members were inside

Suffolk County District Attorney Robert Pascal Biancavilla said Cullen admitted to police that he had killed his mother when he was arrested. He will be prosecuting the trial

Defense Attorney Steve Fondulis, who is representing Cullen, said there was a notion of 'mental illness'

Mrs Cullen worked as an account coordinator with Edwin Bird Wilson, a New York advertising agency in the 1980s.

It is understood that she more recently owned an interior design company in Cold Springs Harbor.

Daily Mail Online tried to call the store on Thursday afternoon but it was closed.

Cold Spring Harbor is a celebrity hotspot and is known for its multi-million dollar estates that dot the Long Island shore.

Among the celebrities who have lived in the area are singer Billy Joel, political commentator Sean Hannity and the late singer Robin Gibb. Joel named his first album after the harbor.

The homicide investigation is ongoing.

Little is known about Cullen's background or what happened in the hours leading up to the slaying.

A silent Denis Cullen led to transport van for arraignment. Police say he intentionally drowned his mom @News12LI pic.twitter.com/RG3bW8mRuQ — Tim Kelchner (@TimKelchner) August 18, 2016