The Los Angeles Police Department said he presented himself as a patient before the shocking attack.
The attacker was finally in custody late Friday after a four-hour-long standoff.
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The suspect was later identified by police as 35-year-old Ashkan Amirsoleymani.
He was arrested on suspicion of attempted murder, and booked on three counts. His bail has been set at $3 million.
The Los Angeles Police Department tried and failed to negotiate with the man who hid out near the emergency room after critically wounding three healthcare professionals and throwing Ventura Boulevard into chaos.
SWAT teams moved carefully as other first responders surrounded the medical center located at 16000 block of Ventura Boulevard where at 3:50 p.m. they alleged the suspect entered the ER and assaulted two nurses and a doctor.
One ultrasound technician saw him moments before the attack.
"He looked high. He just looked very anxious. He was sweating a lot. Half his shirt was drenched with sweat. He had a dog with him," Benjamin Roman said.
LAPD eventually contained the attacker in a room near the ER as they cleared the first floor.
Passersby on busy Ventura Boulevard couldn’t believe their eyes.
"We saw people coming out. Bloody people, kind of. Regular patients too. I think they were kind of evacuating," witness BraydenMordkowitz said.
“I’m praying for all their families. The doctors, the nurses, the patients. I’m disgusted by all the violence that’s been happening lately," witness Mia Abramovitch said.
Dignity Health Northridge Hospital Medical Center said Saturday two of the victims were treated and released. One victim remains in the hospital in fair but stable condition.
"We eventually had to employ some of our other methods outside of negotiations and he was taken into custody. He was being transported to a local hospital for now, for injuries that appear to be self inflicted. He’ll be treated there and subsequently booked," Deputy Chief Alan Hamilton of the LAPD Valley Bureau said at a news conference outside the medical center.
An officer at the scene told reporters the attacker's identity was known to authorities, that he has had several previous run-ins with police and has been arrested twice in the past for assaulting police officers. His most recent arrest occurred last year, police said.
"He’s been arrested for battery on a police officer and resisting arrest in the past here in the San Fernando Valley," Hamilton said.
The LAPD said it doesn’t appear he targeted anyone in particular.
Pat Koren was waiting outside the hospital among the crowd that had formed. She feared her friend was hurt.
"I know she’s working. I see her car. I hope she’s good, and the bad people are going to get caught," she said.
She later confirmed her friend, an ER doctor, was among the three wounded.
LAPD said the attacker acted alone. They recovered the weapon at the scene, described as a knife with a 3 to 4-inch blade.