The Ukrainian immigrant suspected of killing his pregnant wife, 3-year-old son and four other relatives during a murder spree in Sacramento Monday is a "cold-blooded" killer with a criminal background who may be even more dangerous than originally believed, authorities say.

As the manhunt for 27-year-old Nikolay Soltys intensified, the FBI announced Thursday night that it has added the massacre suspect to its Ten Most Wanted list of fugitives at a press conference at the County Sheriff's Office in Sacramento.

Four days after the killings, Soltys was still successfully dodging police, who believe he could still be in the Sacramento area. The authorities have placed 14 of his relatives into protective custody out of fear Soltys' rampage is not finished.

"We are learning more and more how cold-blooded this killer is," Sacramento County Sheriff Lou Blanas said Wednesday. He said anyone associated with Soltys is "at risk of being his next victim."

Meanwhile, relatives offered more details of Soltys' violent past.

In the Ukrainian daily newspaper Fakty, the father of Soltys' wife, Lyubov, said Soltys had attacked her with an ax while the couple still lived in the Ukraine and that Soltys was known as a cruel man who had even been violent toward his parents.

"When we found out Lyubov was killed, the last hope for us was that our grandson was alive," Lyubov's father, Ihor Nakonechnyi, said. "We wanted to take him to our home in the town of Shumsk, since we took care of him since he was a baby, and today we found out our grandson is dead."

The Nakonechnyi family is applying for U.S. visas to attend their daughter's funeral.

Police have learned that Soltys had a history of extortion and racketeering in Sacramento's Russian community, and they say robbery may have been a motive for some of the killings.

"If you look at the handful of his associates and the people he could intimidate for things like welfare money, he could be pretty well set in terms of cash," said Sacramento County Sheriff's Capt. John McGinness.

Soltys was last seen driving an emerald-green and silver Ford Explorer, which police assume bears California plates. Authorities received information Wednesday afternoon that the Explorer had been spotted in the Sacramento area earlier that day, but the sighting was not confirmed.

Soltys switched to the Explorer from his 1995 Nissan Altima, which was found abandoned Monday.

Police said they believed Soltys' reported role in an extortion ring gave him access to large amounts of money and could put him "in a better position than we otherwise believed." That money would also provide him with access to vehicles.

McGuinness said "cash transactions for automobiles are extremely common in immigrant communities, and they're not always that prompt in reporting them."

Someone matching Soltys' description was reportedly sighted in Tennessee Wednesday.

Detectives said Wednesday that Soltys may also have a better command of the English language than originally believed.

Authorities said Soltys has relatives in New York, North Carolina and possibly Oregon and Washington state. A Tennessee truck driver told police he had picked up a hitchhiker Wednesday matching Soltys' description, but authorities seemed to put more stock in the Sacramento sighting.

"We've gotten several hundred tips, only one of which comes from Tennessee," McGinness said Thursday. "We don't want to dismiss anything. But we have obtained information overnight that suggests a greater likelihood he is still in California — specifically in the Sacramento area."

The reward for information leading to Soltys' capture had climbed to $30,000.

Soltys allegedly stabbed his pregnant wife, Lyubov, at their home in a Sacramento suburb before driving the 20-minute distance to Rancho Cordova, where he killed four other relatives — his aunt, uncle and his two young cousins.

Officials said the attack on his aunt and uncle may have been part of a robbery attempt. Many Russian immigrants keep cash in their homes instead of banks, and Soltys may have been after money in the home, detectives said.

Soltys then drove to his mother's home to pick up his son, and he dropped off the Altima and picked up the Explorer. The boy and his father were last seen at 8 p.m. Monday by a man Soltys had asked to work on the Explorer.

Soltys then drove to a remote, wooded area in Placer County. Police say he lured his son into a cardboard box in the woods by placing new toys in the box, then he slashed his son's throat and left him to bleed to death.

Authorities found the boy's body Tuesday afternoon after following directions written in Russian on the back of a photograph of the child and his mother that was left in the Nissan. At the scene, investigators found a set of barefoot adult footprints and a pair of prints of a child's sandals leading away from where the Explorer was parked into the woods, and one pair of adult footprints leading out of the woods back to the vehicle.

Lead Det. Ron Garverick said Wednesday that the boy may have been "violated," but would not elaborate. The coroner's office found no evidence of sexual abuse, but that "there was some bruising on Sergey's lower extremities, back and buttocks, consistent with physical abuse," the Sacramento Bee reported.

In the car, police also found a second note from Soltys with a numbered list referring to the slayings, suggesting each had been killed "for speaking out," Garverick said. "The writing in effect said: 'Number one, this was for Lyubov, for her speaking,' or 'for her tongue.'"

Police had said family members may have chastised Soltys for not having a job or may have spoken about his marital problems outside the family, but Garverick said the meaning of the message was unclear.

Authorities said Soltys' actions show someone acting with cold precision, not someone in a sudden, passionate rage.

Authorities also fear that Soltys' mother and other family members may not be fully cooperating despite the vicious slayings of their loved ones, the Bee reported. Soltys' mother said her son showed no signs of trouble when he picked up his son Monday, but detectives believe Soltys would have been covered in blood after the five stabbings.

The sheriff's command post has been staffed with Russian-speaking deputies and officers from several law enforcement agencies, including the FBI, and has been fielding a flood of calls and potential sightings.

The pattern of slayings — vicious stabbings interspersed with deliberate decision-making — suggest Soltys is delusional, rather than just engaging in crimes of rage or opportunity, said forensic psychologist Barry Rosenfeld, a professor at Fordham University in New York.

"Once there is a cooling-off period in between some of the murders, then it's no longer a spree," Rosenfeld said.

Rosenfeld, the forensic psychologist, says Soltys may have been acting under psychotic delusions that led him to believe there was a rational reason to kill people on his list.

"If he's delusional and has these delusional beliefs, he could have anybody on the list," he said.

A memorial service has been scheduled Sunday for the slain family members at Bethany Slavic Missionary Church outside Sacramento.

The Associated Press contributed to this report.