Officials also said the 35-year-old intended to kill himself when he was found passed out near his gasoline-soaked car on April 21.
Aramazd Andressian Sr. remained in custody following his arrest in Las Vegas on Friday and is expected to be arraigned in Los Angeles later this week, L.A. County Sheriff's Department officials said at a news conference. He was being held on $10 million bail.
Sheriff Jim McDonnell said the elder Andressian was arrested in Nevada because he was "becoming a flight risk." Investigators say that before his arrest in Las Vegas, he was socializing and behaving in a way not consistent of a grieving father.
"He had significantly attempted to change his appearance by shaving his beard and dying his hair," said Detective Joe Mendoza with the L.A. County Sheriff's Department. "We further believe he was making plans to flee the country to a location where he could not be extradited."
Aramazd Andressian Jr., whose body has not been found, was last seen in person when his mother handed him off to her estranged husband for a scheduled visit on April 15.
Investigators would not reveal what new evidence led them to suspect the boy's father, but they believe Andressian senior killed his son in the early morning hours of April 21 after the boy left Disneyland. Investigators say the murder was premeditated.
"He has not been cooperative whatsoever. As a matter of fact, he has given us information in interviews that were untruths," Mendoza said.
Asked about a possible motive for the killing, Mendoza said: "A tumultuous divorce that they were going through at the time."
A statement was read aloud at the press conference from the boy's mother, Ana Estevez.
"It is with deepest and most profound sadness that I want to thank the L.A. County Sheriff's Department, along with the many other agencies and volunteers who have worked and will continue to work diligently to find my son Picqui. In a presentation to the South Pasadena City Council, I shared that it only takes 30 seconds to fall in love with my son. Although most have never met Picqui, it is immensely clear that he has touched your lives in a positive and heartfelt way. My heart is shattered and I will miss my son immensely each and every second of every day for the rest of my life. Picqui was everything great in my life, and I cannot imagine the emptiness and the void that I will bear until we are together again someday. I am eternally grateful for detectives, family, friends and volunteers who loved my Picqui as much as I do."
Although the boy's body has not been found, detectives and the D.A.'s office believe there is enough evidence for a conviction. If convicted as charged, the suspect faces a sentence of 25 years to life in prison, District Attorney Jackie Lacey said.
Investigators say they will resume the search for the boy's body in Santa Barbara, saying they have reason to believe the body was dumped in the area.
Aramazd Andressian Sr. will appear in a Las Vegas court on Tuesday for an extradition hearing.
If convicted as charged, the suspect faces a sentence of 25 years to life in prison, District Attorney Jackie Lacey said.
Police continue to look for the body and want to search some more areas of Santa Barbara County. They believe they left Disneyland together and then went to a lake in Santa Barbara County.
Javier Godinez is neighbors with the missing boy's mother and grandparents. On their street, fliers are posted from the sheriff's department offering a $30,000 reward for information.
"I always used to see him in the backyard with his grandpa and his grandma and his mom, and I'd always hear him laughing and stuff, and now it's like everybody's sad next door," said Godinez.
The child was last seen during a custody exchange with his father. His mother reported him missing on April 22, when Andressian failed to return their son. The 35-year-old was found passed out in a South Pasadena park.
Searches of the park and Cachuma Lake, where Andressian visited on April 21, turned up no signs of the boy.
Andressian was taken into custody but released based on insufficient evidence. Investigators have searched his home and his mother's home.
Andressian's attorney said he was surprised by the arrest on a murder charge and said he's advised his client to waive extradition at Tuesday's hearing.
Police have not disclosed what information led them to make the arrest or why Andressian was in Las Vegas. They said they had presented evidence in the case to the Los Angeles County District Attorney's Office and obtained a murder filing against the suspect.
The 5-year-old was last seen early Friday, April 21, in his father's custody.
Andressian Sr.'s gray 2004 BMW sedan was found with its interior and exterior doused with gasoline. He could not account for his son's whereabouts.
MORE: Dad of missing South Pasadena boy says he was possibly 'attacked' at park
Since that time, authorities have mounted massive search operations, scouring local parks and going as far north as Santa Barbara. They also executed a search warrant at the home of the mother of Andressian Sr. in Montebello.
The arrest was made by officials with the Los Angeles County Sheriff's Department, the Las Vegas police and the U.S. Marshals Service.