An Arizona couple who police named persons of interest in the case of a missing baby took polygraph tests on Friday, MyFoxPhoenix.com reported.

Jack and Terri Smith spent hours at the Tempe, Ariz., police station answering questions as part of the police investigation into the disappearance of 8-month-old Gabriel Johnson.

Police said Thursday they believe the Smiths, who had wanted to adopt Garbriel, may know more about the missing child's whereabouts than they're telling investigators.

The baby was last seen in San Antonio in late December with his 23-year-old mother, Elizabeth Johnson.

Johnson was arrested last week in Florida on suspicion of custodial interference after she didn't show up for a custody hearing in Arizona. Her car was later found by the FBI in San Antonio.

She has since told a Phoenix television station that she gave the boy away in San Antonio. She made the statement after she sent a text message to her ex-boyfriend telling him she had killed the baby.

Police believe the baby has not been harmed.

"We are getting some indications that Gabriel is alive," Carbajal said Thursday. "We can't say specifically just what those are, but we are getting some indications that those are correct."

The Smiths have given numerous media interviews in recent days where they said they met Johnson at an airport during a long layover and befriended her. Terri Smith said the young mother wanted to give up her baby for adoption but the ex-boyfriend wouldn't go along.

Terri Smith told KTVK-TV in Phoenix on Thursday that the couple has cooperated fully with police and have nothing to hide.

The baby's father, Logan McQueary, has said Johnson tried several times to get him to sign away his parental rights but that he refused.

Tempe police have been inundated with tips and are now directing callers to the National Center for Missing or Exploited Children.

Carbajal urged anyone who has the baby to bring him to a safe place like a fire station or a hospital if they are afraid to come forward.

"Our focus has always been the same, and that is locating Gabriel," he said.

Click here for more from MyFoxPhoenix.com.