Brad Garrett, a former FBI special agent and a consultant to ABC News, said the first decision police face is whether they can eliminate the parents from suspicion. "Sometimes there's not a clear line you can draw," he said.
In these cases what ensues, he said, is "a delicate balance to push parents to tell the truth while not alienating them so they stop cooperating." The key is "good cop-bad cop," a perfect illustration of the aforementioned dual role. One detective stays at the parents' side, protecting them. Another challenges and provokes them. Then they compare notes and look for lies and inconsistencies.
"[The police] don't like doing that; they just have to do it when there's no evidence [leading elsewhere]," Garrett said.
In a recent interview with "Good Morning America" Garrett called police's focus on Irwin's parents "logical." He said Lisa was "probably not" abducted by a stranger -- "statistically and otherwise, it doesn't really fit" -- but an acquaintance, plumber, neighbor or babysitter was more plausible.
Garrett said sometimes missing-baby cases happen when the baby dies by accident and the parent or parents panic and dispose of the body. Usually the disposal is shoddy and the body is found fairly quickly. Therefore, the fact that Lisa is still missing makes it less likely that, if she is dead, she died accidentally, he noted.
"What you're left with is not being closed-minded," Garrett said. "It's easy to put blinders on, the longer it goes on."
The Aisenbergs live in Bethesda, Md. Steve delivers high-tech equipment to ophthalmologists, and Marlene is a real estate agent. "Our children are in college. They're growing up to be great people. We're very proud of them," said Steve. They celebrate Sabrina's birthday. They continue to seek her in various ways. "But we try not to let [Sabrina's absence] consume us," Steve said.
The Aisenbergs still get leads -- there have been eight since March, Marlene said – directly and through the National Center for Missing and Exploited Children, which assists in their search.
"Unfortunately, a lot of them are ones that people have called in before," Marlene said.
"It's been 14 years. We believe she's still alive. It just needs to be yesterday, you know?" Marlene said. They take hope from the story of Elizabeth Smart, who in 2003 was rescued after being abducted for nine months. "It only takes that one lead," said Marlene.
The Aisenbergs also have a private investigator trying to find Sabrina.
In 2003 an Illinois girl was thought perhaps to be Sabrina, but a DNA test was negative. Marlene said there were two promising leads on girls who might be Sabrina, one in California.
Time could be on their side. "Sabrina's 14 now," Steve said. "In a year or two, she'll want a driver's license," for which she will need a birth certificate. But "if someone is willing to take a child, they're willing to create false documentation," he said.
Marlene said they didn't have much contact with the Hillsborough County Sheriff's Office. But, Steve added, they believe it will be with their help that they will be reunited with their daughter. If the crucial break occurs, "they're going to be the ones to [follow it through]."
As to Lisa Irwin's parents, beyond expressing his and Marlene's support for them, Steve said, "Right now what this family needs most is the support of their community." When asked if that was missing when they were in the spotlight now pointed at Irwin's parents, he replied, "It was there, but it also got squashed at certain intervals."
Anyone with information about Sabrina is requested to call the Hillsborough County Sheriff's Office at 813-247-8200.
Anyone with information about Lisa is requested to call the Kansas City Police Department at 816-474-8477.