Detectives believe a five-year-old California boy who has been missing for two months was killed by his father after a family trip to Disneyland. But despite weeks of intensive searches, investigators have not yet found the boy's body.

Still, prosecutors say they are confident they can secure a murder conviction.

Legal experts say so-called “no body cases” can result in convictions, but they present an additional challenge for prosecutors: proving the victim is actually dead.

“In most homicide prosecutions, the fact the person died is not the issue,” said Heidi Rummel, a law professor at the University of Southern California and former federal prosecutor.

In the vast majority of murder cases, proving someone was a homicide victim is relatively easy with an autopsy, but without a body, prosecutors will need to prove the case with only circumstantial evidence, Rummel said.

The boy's father, Aramazd Andressian Sr, was arrested Friday in Las Vegas and is being held there on $10 million bail. He is scheduled to appear for an extradition hearing in Las Vegas on Tuesday and is expected to return to Los Angeles by the end of the week, prosecutors said.

Los Angeles County District Attorney Jackie Lacey said she understands it seems unusual to file a murder charge when investigators haven't found the boy's body and concedes it will be a challenge. But Lacey said she's confident prosecutors will land a conviction in the case.

“As time goes on and there's no sighting of a person, of the victim, obviously a case gets a bit stronger, but it just forces us in law enforcement to work harder to look for evidence that would indicate the person is guilty of murder,” Lacey said. “I would not have authorised the filing of this case unless I believed that a jury would hear the evidence and convict the defendant of murder.”

Investigators have been searching for the missing boy since his father was found passed out in a large park in South Pasadena, California, on 22 April. Sheriff's officials say the father had taken prescription pills and was found in a car doused in gasoline in what they say was an attempt to take his own life.

The boy was last seen leaving Disneyland with his father around 1am on 21 April. Investigators believe Andressian killed his son a short time later and then drove about 145 miles (230 kilometres) later that morning to Lake Cachuma in Santa Barbara County. Sheriff's homicide detectives have searched the lake twice in the past few months, using dogs and a dive team to help in the search.

Detectives said they believe Andressian killed his son in an attempt to get back at his estranged wife for their tumultuous relationship, Los Angeles County sheriff's Lieutenant Joe Mendoza said.

Sheriff's officials and federal agents arrested Andressian because he was “becoming a flight risk,” Los Angeles County Sheriff Jim McDonnell said. Investigators believed Andressian was planning to leave the US and flee to a country that does not have an extradition agreement with the US, but McDonnell declined to name that country.

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Andressian had also changed his appearance by dying his hair and shaving off his facial hair and had been socialising while living out of a Las Vegas hotel for 47 days, conduct characterised as inconsistent with that of a grieving parent, Mendoza said.

Andressian's attorney, Daniel Nardoni, has said his client “is adamant that he never harmed his son Aramazd and is innocent of the charges.” Nardoni did not immediately respond Monday to an emailed request for comment.