Advised by the stalking unit of the potential danger, Spears got a restraining order. When Shizawa's tourist visa expired, that restraining order was used to get him on a no-fly list. He's since tried to reenter the United States five times and each time he has been stopped.
"Typically if you dig into these cases you find much more disability and disorder than how it appears on the surface," said Reid Meloy, who is widely regarded as one of the nation's experts on stalkers, particularly those who threaten celebrities.
"Celebrity stalkers tend to, as a group, to be much more mentally ill than other stalkers," said Meloy.
Meloy says the average celebrity stalking case lasts about 16 months, but he's seen them go on as long as 15 years.
While the risk of "death by stalker" is relatively low for celebrities, Meloy says the frequency of homicidal celebrity stalking has increased significantly since 1980.
"Violence among celebrity stalkers, if it is carried out, is usually planned and purposeful," he said.
Rebecca Schaeffer, John Lennon and Jill Dando -- murdered
Jerry Lewis, Monica Seles and Theresa Saldana -- stabbed
Michael Landon's stalker killed two security guards
Olivia Newton John's stalker killed five members of his family
PPaula Abdul's and David Letterman's stalkers both committed suicide.
"Their lives are bereft of any normal satisfactions," said Meloy. "Oftentimes they are living alone and have had very unstable work history and have not done well and oftentimes because of the psychiatric disorder, but paradoxically they tend to be brighter than average so you see an ability to manipulate that is better than average criminal behavior."
One stalker, considered to be extremely dangerous by both Meloy and LAPD, was Jonathan Norman.
"Mr. Norman had a plan to kidnap Steven Spielberg and to sexually assault Steven Spielberg," said Dunn. "We deal with delusional people all the time with celebrity cases. … But when they start acting on their impulses, as Jonathan Norman did with Steven Spielberg and they start trying to put a plan into action, that's when the hairs on the back of your neck have to stand up."
Investigators say Norman tried to pass himself off as Spielberg's adopted son, but his journal entries later revealed a darker obsession with the Hollywood icon.
"We pulled some of the pages from his handwritten notes and what's clear here is the planning," said Meloy, reading parts of Norman's entries.
"I would force him to put on a screening muzzle," Norman wrote, "and then put handcuffs on him with his hands in front of him I would then have him direct me to his apartment and then cuff and gag any roommates he might have."
But it didn't stop there. Norman had a shopping list.
"He is going to have eye masks, three cuffs, four pairs of nipple clippers, dog collars, check for nipple shockers," said Meloy as he read from Norman's own lists that were eventually used as evidence in the case. "Three locks with same key and again, you see both the level of aggression and sexual aggression that Norman thought about and likely fantasized about as part of his attack on Spielberg."
Norman was sentenced to 25 years to life for his crimes. But even when the threats don't reach this level, celebrities often live in fear.