Federal authorities are helping Oklahoma police investigate the shootings of five of African-Americans, three of whom were killed, within a few hours.
Three men and one woman were shot within 1.6km of each other in north Tulsa at around 1am local time on Friday morning, police and community members said.
Police said that a fifth victim, 31-year-old William Allen, whose body was discovered outside a nearby funeral home around 8am on Friday, was likely shot at about the same time as the others.
Each of the victims were African-American, but Chuck Jordan, Tulsa police chief, said it was too early to know whether the shootings were racially motivated, and police have not yet been able to prove forensically that the shootings are linked.
'Hate crime issue'
Based on a description of the shooter by one of the survivors, police believe the suspect to be a white man driving a white pickup vehicle.
"The whole race issue, the hate crime issue, there's a very logical theory that would say that's what it could be, but I'm a police officer, I've got to go by the evidence," Jordan said.
"Its just not time for us to say that," Jordan said. "Right now I'm worried about more of my citizens being murdered."
Police identified the other people killed as Dannaer Fields, 49, and Bobby Clark, 54.
The names of the two survivors were not released and authorities said only that they were expected to survive.
More than two dozen officers are investigating the case, along with the FBI, the US Marshals Service and other agencies, Jason Willingham, a police spokesperson, said.
'People are fearful'
As investigators searched for the killer, the attacks sparked anxiety among Tulsa's black community, leaving many people worried that the shooter, or a copy-cat criminal, will continue the assault.
"People are fearful ... . They are afraid they can't walk down the street," said Jack Henderson, Tulsa city councilman who represents the district in which the shootings took place.
Authorities asked people to come forward with any information on the shootings.
"All citizens of Tulsa understand the significance of this event," Dewey Bartlett, Tulsa's mayor, said.
But the Reverend Warren Blakney Sr, the local chapter president of the National Association for the Advancement of Colored People, a civil rights organisation, said "avid distrust" between the black community and the police department had raised concerns that the shootings would not be fully investigated.
"We have to handle this because there are a number of African-American males who are not going to allow this to happen in their neighborhood," he said.
"We're trying to quell the feeling of `let's get someone' and we will make as certain as we can that this isn't pushed under the rug."
Tainted police record
Tulsa's police department has been tainted by accusations of corruption.
Three ex-police officers and a former federal agent were sentenced to prison in December after a two-year investigation involving allegations of falsified search warrants, nonexistent informants, perjury and stolen drugs and money.
Two other ex-officers were acquitted of stealing money during an FBI sting and fired after an internal affairs investigation.
More than a half-dozen lawsuits have been filed by people who claim they were wrongfully locked up by police, and nearly 40 people had their convictions overturned or prison sentences commuted as a result of the corruption probe.
Prosecutors have suggested the five police officers who were charged were part of a broader plot in which corrupt officers stole money and drugs, conducted illegal searches and fabricated evidence without fear of getting caught.