As of Saturday, Jeremy Joseph Christian was being held on suspicion of state crimes, including aggravated murder, for which the death penalty is a possible sentence.
The FBI has joined the Portland police-led investigation to gather evidence.
"It's too early to say whether last night's violence was an act of domestic terrorism or a federal hate crime," Renn Cannon, the special agent in charge of the Portland office of the FBI, told reporters Saturday.
The killings -- in which two men were fatally stabbed and another was wounded -- came after Christian allegedly started yelling what "would best be characterized as hate speech toward a variety of ethnicities and religions" toward two women in a Metropolitan Area Express (MAX) light-rail train, Portland police Sgt. Pete Simpson said.
Witnesses described the women, one of whom was wearing a hijab, as Muslim.
As two men tried to calm him down, Christian, 35, stabbed them and one other man, police said. The two men who died were identified Saturday as Ricky John Best , 53, of Happy Valley, and Taliesin Myrddin Namkai-Meche, 23, of Portland.
Best died at the scene, and Meche died at a hospital, police said.
Best, an Army veteran and father of four children, was a "model public servant" working for the city of Portland, a spokesman for Portland Commissioner Chloe Eudaly said.
"My heart goes out to the family of Rick Best. They have lost a husband and a father, and we have lost a treasured employee," Eudaly said in a statement.
Namkai-Meche graduated with a degree in economics from Portland's Reed College last year. In a statement, his thesis adviser Noelwah R. Netusil, said he was "a very caring person, smart, hardworking, and with such a bright future."
The third man, Micah David-Cole Fletcher, 21, is still at a hospital with serious but non-life-threatening wounds, police added.
Christian was charged with two counts of aggravated murder and one count of attempted murder, all felonies. He also was charged with misdemeanors: two counts of second-degree intimidation and a count of being a felon in possession of a restricted weapon, police said.
His felony convictions -- for robbery and kidnapping -- came in 2002, state corrections department records show.
He was being held Saturday at the Multnomah County jail without opportunity for bail. His arraignment was set for Tuesday in county court, police said.
"We don't yet know what angle the investigation is going to take," Portland police Chief Mike Marshman said.
Confrontations last month
Christian appeared to cause a stir at a rally for free speech in Portland last month, shouting at people and at one point blurting a racial epithet, videos from reporter Doug Brown of The Portland Mercury
weekly newspaper show.
Christian, wearing a cap and draped in a Revolutionary War-era flag, identifies himself in one of the videos from the rally at Portland's Montavilla Park
on April 29.
One video shows him arriving with a baseball bat, which he handed over to one of the police officers who approached him.
The videos show him shouting at people, at one point saying the N-word, as police officers separated him from others.
"I'm a nihilist," he says at one point. "This is my safe place."
Saturday, police said detectives are looking at Christian's background, "including the information publicly available about the suspect's extremist ideology."
The stabbings happened hours before the start of Ramadan
, a month long Muslim holy period of prayer, fasting and charity. Portland, Oregon's most populous city with 640,000 residents, has about 50,000 Muslims, The Oregonian newspaper reported.
Police locate women
Police have contacted the women, who left the scene, but Simpson wouldn't discuss whether they have been interviewed.
He said detectives want to interview others who took off. It is unclear how many people were on the train, but officials said at the time of day it would have been crowded.
The victims apparently didn't know Christian and were trying to protect the women, police said.
Witnesses said one was stabbed in the neck, CNN affiliate KOIN
'Hate is evil'
Following the attack, the Council on American-Islamic Relations called on President Donald Trump to denounce "rising bigotry" and acts of violence against Muslims.
"President Trump must speak out personally against the rising tide of Islamophobia and other forms of bigotry and racism in our nation that he has provoked through his numerous statements, policies and appointments that have negatively impacted minority communities," CAIR National Executive Director Nihad Awad said.
Portland Mayor Ted Wheeler condemned the incident and addressed Portland's residents of all races and ethnicities saying local officials are working to ensure their safety.
"We cannot abide bigotry or racism or xenophobia. We can't never tolerate violence, we have to come together in love one another," Wheeler said during a news conference.
Sen. Jeff Merkley of Oregon took to Twitter
to decry the stabbings.
"Terrible tragedy on Portland's Max Train," he posted. "Champions of justice risked and lost their lives. Hate is evil."
Gov. Kate Brown called for Oregonians to come together.
"Let's not let hate and fear divide us. Instead let's take the example of the good Samaritans who sacrificed their lives for the safety of others and unite for a kinder, gentler Oregon," she said.