An Arizona man accused of murdering a former coworker he stalked at the gym kept a sick notebook detailing his obsession.

The journal belonging to Juan Cuellar, 51, was uncovered by police investigating the death of Doris Aguilar, 25.

Both were found dead in a car in outside a Phoenix steakhouse on July 7, a week after Aguilar was reported missing by her panic stricken family.

Police said Cuellar had killed her before shooting himself following what her family say was a campaign of harassment lasting more than a year.

The notebook contains chilling entries which shed further light on the extent of Cuellar’s obsession, although investigators noted that many of the entries were false.

‘It took a little bit to get through the notebook, which offers a whole different dynamic of what happened than maybe what some of the evidence showed,’ said Phoenix Police Sergeant Rob Scherer, told AZ Family.

One of the entries appears to reference the killing and make out like it was accidental.

 ‘She picked it up. She didn’t know that the safety was off. I went to grab the gun, and it went off. I panicked and left. I knew it was going to be my fault,’ Cuellar wrote.

However, police refuted the statement saying the weapon found inside the vehicle does not have a safety.

Chillingly, he then details driving around with the victim and changing her clothes so she wouldn’t be dirty.

‘I couldn’t bear to be without her in my life,’ Cuellar wrote in another entry, which reportedly suggests he was angry about her seeing other men.

 ‘I hope she can forgive me. Please tell my family it was an accident,’ one of the last entries reads.

Police discovered a notebook filled with chilling entries written by her alleged killer Juan Cuellar which detailed his sick obsession

Her body was discovered along with Cuellar’s inside a car parked outside this Phoenix steakhouse

Other bizarre writings included delusions about a daughter the two shared who does not appear to exist.

‘I can tell you there was no element of a romantic relationship at the time of the homicide,’ Scherer said. ‘We don’t have any information of a child in common between the two,’ said Scherer.

Aguilar was reported missing on July 1 after she failed to turn up to the gym where she spent a lot of her time.

Her family immediately suspected her former co-worker Cuellar might be to blame after he sent her harassing messages.

The two met while working at a distribution center before Cuellar was fired over his threatening behavior. 

The stalker would show up in the parking lot of the gym Aguilar attended, forcing her to switch gyms three times, according to a private investigator the family hired to try and trace her.

Just a few days before Aguilar mysteriously disappeared, on June 27, the alleged stalker seemed to indicate he knew which facility she was now attending.

Her concerned family had reported her missing a week prior and immediately suspected Cuellar might be involved after he had stalked her at several gyms

On June 27, just a few days before Aguilar mysteriously disappeared, an alleged stalker wrote on a Facebook page with a gun as its profile picture that he was getting his membership back – presumably to the gym that Aguilar would frequent

He wrote on a Facebook page with a gun as its profile picture: ‘Getting my membership back… [see] you soon,’ Fox 10 reports.

Aguilar’s car was eventually found abandoned on July 8 at a shopping mall near the intersection of Interstate 17 and Bethany Home Road.

The car was unlocked, with her purse and wallet inside full of cash.

Police said that Cuellar killed Aguilar before taking his own life a few days later, although they cannot be sure where exactly she died.

Their badly decomposed bodies were later found in the same vehicle. 

Aguilar’s family say Cuellar’s obsession is what drove him to murder.

‘They have their belief as the why. Our investigators are still trying to get to that,’ Scherer said. 

Aguilar’s family and Steve Fischer, a private investigator, believe that Cuellar had been Aguilar for over a year.

The notebook contained some entries which were false, including delusions about a daughter the two shared together who does not appear to exist

Private investigator Steve Fischer said the stalking had been going on for more than a year

According to Fischer, Cuellar was so obsessed with Aguilar that he kept showing up at the gyms where she worked out.

‘He was obsessed to the point where he had a picture of her taped to the back of his cell phone,’ Fischer said.