Another male victim was injured and is in serious condition in the ICU

Three people were killed and one person was injured during a mass shooting at a house party in an affluent suburb near Seattle.

A shooter walked into a party 'full of his friends' in Mukilteo and fired around 20 bullets before he fled the scene just before 12.30am on Saturday. One of the victims is his ex-girlfriend.

Allen Ivanov, a 19-year-old incoming sophomore at the University of Washington, is in custody, Mukilteo Officer Myron Travis confirmed to Daily Mail Online.

Anna Bui, Jake Long and Jordan Ebner were killed in the shooting. They appear to have all been classmates of Ivanov's at Kamiak High School.

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Mukilteo police have confirmed to Daily Mail Online that Allen Ivanov is in custody following a mass shooting in the affluent Seattle neighborhood of Mukilteo that cost the lives of three young people on Saturday morning

Anna Bui has been confirmed as one of the three victims. She and Ivanov reportedly broke up last week

Jordan Ebner, a student at Everett Community College, also lost his life in the horrific mass shooting

Jake Long (pictured with his girlfriend at prom) was the third victim of the tragedy that has rocked Mukilteo

One person was also injured when a gunman walked into a house party filled with up to 20 young people and fired at least 20 bullets before fleeing the scene. Pictured are parents waiting to hear information

The neighborhood was sealed off folowing the shooting and a local church has opened as a center for parents and relatives to reunite with their children

Ivanov was arrested over 100 miles away in Lewis County when officers spotted a car that matched the suspect's description about two hours after the shooting.

Officer Myron Travis said police are not looking for any other suspects.

The shooter was due to be be booked into the Snohomish County Jail on Saturday on murder-investigation charges, according to the Seattle Times.

Ivanov left cryptic tweets on his Twitter account just two days before the shooting.

'First and last tweet,' he wrote on July 28. 'I've been through it all.'

He then tweeted 'What's Ruger gonna think?', which seems to be a reference to Ruger Firearms.

Ivanov, who graduated from Kamiak High School in 2015, also posted a photo on Instagram this week that showed a rifle on the ground, with three bullets on the side.

On the rifle is a sticker from the exclusive Supreme New York streetwear brand that reads: 'Supreme 16 You Still Suck'.

There is also a piece of blue painter's tape on the gun that reads 'Christopher', which public records show is Ivanov's middle name.

The caption of the post reads: 'You can't run with me.'

Ivanov is a computer science and engineering major, according to his LinkedIn page.

He is also an engineer and founder of Skirmos, an open source laser tag system that allows users to 'imagine your favorite first-person video game in real life'.

A Kickstarter campaign raised more than $90,000 for the factory prototypes.

Ivanov was also currently working at the Genius Bar for an Apple store and had previously worked as a sales associate at Abercrombie & Fitch.

Mukilteo resident Susan Gemmer said the shooter and one of the victims had just broken up last week, she revealed to KOMO. It has since been revealed that Bui was the ex-girlfriend.

Gemmer said her 18-year-old granddaughter Alexis had been at the party, which was attended by Kamiak High graduates aged 18 to 20, when the shooter arrived with a rifle.

Alexis revealed that the gunman shot two people at a fire pit in the home's backyard before he went onto the roof, where some of the guests were hanging out, and opened fire.

The young man who lived at the home tried to lead Alexis to safety out of his garage door, but was shot as he exited, Gemmer told the station.

He was able to make it across the street to safety, while Alexis ran back to the home and hid in a closet.

'We jumped out of bed, got dressed and got in the car as fast as we could,' Gemmer said.

Ivanov posted a picture of this rifle, with three bullets on the side, on his Instagram account just days before the shooting. The rifle has a sticker from exclusive New York streetwear brand Supreme

Ivanov, a University of Washington student, also posted a cryptic tweet on Thursday

Ivanov also posted this tweet, which seems to be a reference to Ruger Firearms. A friend commented on the tweet in the early hours of Saturday morning after the shooting

Authorities said witnesses gave information and description details that helped police catch their suspect after he fled the $1million home in a 2016 Subaru WRX.

Ivanov had recently posted multiple pictures of a Subaru WRX on his Instagram.

The shooting has rattled the tiny town of Mukilteo, where teens often walk on graduation day with the same people they began kindergarten with.

A Kamiak high school graduate, who asked not to be identified, told Daily Mail Online that her younger sister and Ivanov had been very close in elementary school.

Although they didn’t stay in touch as they grew older, she said Ivanov was known to be part of the popular crowd.

‘He was wealthy and smart and I’m sure those younger girls found him attractive,’ she said.

‘I can’t speak on what may have been going on in his home life or inside his head, but externally, he seemed to be doing pretty well.’

The graduate, who is two years older than Ivanov, said the town was proud of his recent success and had been 'rooting for him' and what they saw to be a 'strong future'.

But that all clearly changed early Saturday morning.

Ivanov's LinkedIn page said he is an engineer and founder at Skirmos, an open source laser tag system

The 'Skirmos experiences' (pictured) allows users to experience their 'favorite video shooter game in real life'

Authorities said the suspect fled the $1million home in a 2016 Subaru WRX. Ivanov had recently posted multiple pictures of a Subaru WRX (pictured) on his Instagram

‘I don’t want to get this twisted and be painting him as this nice young man. He’s not. Obviously not. He’s a monster,’ she said.

‘And no amount of money or success can negate that.’

Dave Wakeman, who is listed as one of the founders of Skirmos, told Daily Mail Online the company was shocked by the shooting.

'Allen Ivanov has worked with us and been our friend and colleague for a number of years,' he said.

'That said, this event is another example of our need for gun reform in this country.'

'No person should have the opportunity to possess a firearm that can discharge a weapon 20 times in short succession.'

'The time to take action so that senseless deaths like this don't continue to be the norm in our country has passed. We absolutely need to take immediate action on this issue.'

Friends of the three young victims immediately took to social media to pay tribute.

The Kamiak graduate told Daily Mail Online that she had a class with Bui, who she described as the ‘sweetest girl’.

‘She always had a smile on her face and a joke at the ready,’ she said.

‘She had so much energy and a light about her that could just brighten up a room. If she was in the building, you could hear her laughter.’

‘She cared so much about her friends and was so full of love. She had a huge heart.’

Kayla, a senior at Kamiak, said Anna had been her idol both in choir and life itself.

'She was the kindest and happiest soul,' Kayla told the Daily Mail Online.

Members of Kamiak Choir organized a candlelight vigil at the high school on Saturday night to honor Anna, Jake and Jordan.

Mukilteo's official Twitter feed reported three fatalities in the waterfront Chennault Beach neighborhood, about 25 miles north of Seattle.

Bui has been described as the 'sweetest girl' with the 'kindest and happiest' soul by her former classmates

Jordan (left in the black suit) was a student at Everett Community College. Jake (right) is being remembered by friends as a great baseball player

The house party was taking place at this $1million residence in the waterfront Chennault Beach neighborhood, about 25 miles north of Seattle

Investigators were still at the scene of the triple murder on Saturday afternoon

One person was taken to Harborview Medical Center and is listed in serious condition in the ICU at the Harborview Medical Center in Seattle.

Mukilteo Mayor Jennifer Gregerson said initial reports regarding the injured victim are 'hopeful'.

‘Our community has been shaken to its core,' she said during a media briefing. 'We grieve with the families of those lost in this horrible event. We will stand with them and be here for them.’

‘One thing is clear. Our community has suffered a great loss tonight. There were many young people who saw and heard things that no one should ever have to experience.'

'I hope that we can all lift them up in our thoughts and give them space to mourn and grieve and begin the difficult process of recovery.’

Parents could be seen at the scene hoping to find more information about their children.

The neighborhood was sealed off following the gunfire and a local church opened as a center for parents and relatives to reunite with their children.

Police interviewed the party guests before releasing them to the church.

Many local and nearby residents took to Twitter on Saturday morning to express disbelief that such a violent crime had occurred in the peaceful neighborhood.

'Of all places for this to happen, you'd never think Muilteo...damn,' wrote Brandon Gustafson.

There is no known motive at this time. The investigation is ongoing.