(CNN) -- An Arizona couple who wanted to adopt a woman's baby know more than they're saying about the 8-month-old boy's disappearance, police say. 

Gabriel Johnson hasn't been seen since December 26, police said, and his mother was arrested last week in Miami Beach, Florida, after not reporting for a December 28 child custody hearing in Arizona. 

"We have some good indications at this point with our investigation that Tammi and Jack Smith do know more information than they have provided to us that could possibly lead us to Gabriel," Sgt. Steve Carbajal, spokesman for the Tempe (Arizona) Police Department, told HLN's Nancy Grace on Thursday. 

For their part, the Smiths told HLN's Jean Casarez on "Nancy Grace" that they voluntarily took polygraph tests Friday at the Tempe Police Department. 

"The polygraph test is not like what you see on TV," Jack Smith said. "It's almost like being strapped into the electric chair, so it's very intensive. And we were very happy to do it." 

The Smiths have said they met Elizabeth Johnson, 23, seven months ago during a long layover at an airport, and later the three discussed a plan to adopt Johnson's child, Gabriel. 

But the boy's father, Logan McQueary, has legal custody and has said Elizabeth Johnson urged him to sign papers giving the Smiths custody of the boy, but he refused. 

"She didn't want Logan to have the baby, and we couldn't adopt the baby because Logan wouldn't sign the papers," Tammi Smith said on "Nancy Grace." 
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But the boy's father, Logan McQueary, has legal custody and has said Elizabeth Johnson urged him to sign papers giving the Smiths custody of the boy, but he refused. 
