Daniel Stillwell: Man who killed ex-girlfriend, another man in double homicide found dead after driving car over cliff

A 23-year-old student confessed to killing his ex-girlfriend and her new boyfriend, then drove to his death off a cliff known as Devil's Backbone.

Hailey Nicholls, a 22-year-old fashion merchandising student, and Jesse Robledo, 26, were shot dead in her San Marcos apartment just before 1am on Thursday.

Police mounted a manhunt for Daniel Brewster Stillwell, who had previously dated Miss Nicholls, but he apparently killed himself before they could reach him.

Victim: Fashion student Hailey Nicholls was shot dead by her ex-boyfriend

Murder: Daniel Brewster Stillwell, left, killed Hailey and Jesse Robledo, right, before taking his own life

According to his arrest warrant, when Stillwell saw Mr Robledo's car outside the home of his ex-girlfriend, he took his roommate's gun and broke into her apartment.

He used a brick to break through a glass door before shooting Mr Robledo and then Miss Nicholls in her bedroom, according to KVUE.

The double murder took place at the Executive Townhouses complex, where Stillwell and his ex both had apartments.

'Saying more than I probably should, I don't believe this was a rise of sudden passion. I believe this was planned,' San Marcos Police Chief Howard Williams said.

Stillwell, who had worked at a branch of Chipotle with his victims, was said to be controlling of Miss Nicholls, according to The University Star.

Shot: Restaurant worker Jesse Robledo was found dead in Hailey's bedroom

Promising: Hailey, 22, was a fashion merchandising student at Texas State University

Investigation: Stillwell's body was found in his car in a remote region known as the Devil's Backbone. It is not yet known how he died

Fellow student Sarah Wolfshohl said her relationship with Stillwell ended last week, but Miss Nicholls had been trying to leave him for some time.

'I never thought he was capable of taking it to that point,' Miss Wolfshohl said, adding that Stillwell could be intense and wouldn't let Nicholls go out.

Miss Nicholls, who had a twin sister, Shelby, was studying at Texas State University, where Stillwell also took classes.

Her ex-boyfriend is said to have called his roommate, Daniel Trottier to confess, and told him that he planned to take his own life.

Stillwell told him 'he did not want to live in a cage and was going to kill himself by driving off a cliff,' Statesman reported.

Neighbor Cecilia Rodriguez told Austin YNN she called police after hearing gun fire.

'I heard some more what I thought was gun shots and it just got quiet after that and all I heard was a bit of shuffling as the person ran out,' she said.

Loss: Hailey, pictured with her twin Shelby, was found dead in her apartment

Police tracked Stillwell by following his cell phone records, until his phone cut off at about 2am. His body was found inside the wreckage of his car at about lunchtime, near a spot known as Devil's Backbone.

'We're not sure exactly whether the collision killed him or if he shot himself,' Police Chief Williams said.

A post-mortem examination is being carried out, but police said it was apparent that he died from self-inflicted injuries'.