Oklahoma plans to file a murder charge as early as Tuesday against Alton Nolen, who is suspected of beheading a former co-worker at a food distribution center last week, prosecutors said.
The Federal Bureau of Investigation is also looking into Nolen's background for any potential religious ties to the attack after former colleagues said he had attempted to convert them to Islam. Police said Nolen had recently become a Muslim.
Nolen, 30, is suspected of attacking two women shortly after being fired on Thursday at Vaughan Foods in Moore, a suburb of Oklahoma City, police said.
"The District Attorney's office anticipates a charging decision in the Alton Nolen case early Tuesday afternoon," it said in a statement.
Police said he stabbed and then severed the head of 54-year-old Colleen Hufford and attacked 43-year-old Traci Johnson before being shot by Mark Vaughan, the company's chief operating officer who is also a sheriff's deputy.
Johnson is expected to recover, police said.
The FBI investigation comes as Islamist militants fighting in Iraq and Syria have released videos that purported to show the beheadings of two U.S. journalists and a British aid worker.
Police said a Facebook page for Nolen, under his Muslim name Jah'Keem Yisrael, contained anti-American and pro-Islamic jihad comments.
A woman who said she was Nolen's mother took to social media to apologize to the victims of the attack.
"My heart is just so heavy right now," Joyce Nolen said in a Facebook posting over the weekend.
"I want to apologize to both families, because this is not Alton."
Megan Nolen, the suspect' sister, said in the same video that Alton was not a violent person.
Alton Nolen underwent surgery at a local hospital after being shot and was expected to survive, Moore police said.
Nolen had a non-violent criminal record, including drug-related arrests, a jail escape and resisting police, authorities said.
A group of Oklahoma lawmakers in the heavily Republican state issued a statement that suggested the attack might have been related to militant Islam and not sparked by a workplace dispute.
"Unfortunately, jihadism is no longer confined to foreign soil. We must be alert to it here and take steps to counter the doctrine, institutions and organizations that foster it," the Counterterrorism Caucus of the Oklahoma Legislature said.
(Writing by Jon Herskovitz; Editing by Doina Chiacu and Eric Walsh)