Where's Baby Gabriel?

INSIDE EDITION reports on a twisted story of missing 8-month-old baby boy, a mom going to jail, and a father frantic to find his son.

It's a mystery gripping the nation: Where is baby Gabriel?

His mother is in jail and claims she gave him away to complete strangers.  Her own father says she's lying:

Baby Gabriel's grandfather said "I think she has given the baby to somebody.  I think she knows who it is."

Gabriel is at the center of a bitter custody battle, and his distraught dad, Logan McQueary, is begging for his safe return.

On Good Morning America, Logan said "We're just asking please, I want my son back."

Only Gabriel's mother Elizabeth Johnson knows what really happened to him.  She made a bizarre cross-country trip with Gabriel, leaving her home in Tempe, Arizona, a few days before Christmas and driving to San Antonio, where she says she met two complete strangers in a park and gave Gabriel to them.

The FBI found Johnson's car in San Antonio with the empty infant car seat still in it.

Johnson then traveled alone to Miami beach where she was arrested December 29th. 

She admits sending a chilling text message to the baby's father saying, "There is no way in hell you will ever see your son again.  I killed him."

She now says she never harmed Gabriel and was only trying to upset the father. 

Gabriel's grandfather said, "She discovered after 8 months that there’s more to this parenting than she wanted to get involved in.  She's into fashion."

Now there's a new twist.  What does an Arizona couple know about what happened to baby Gabriel?

Jack and Tammi Smith wanted to adopt Gabriel, but police today labeled them "persons of interest" and say they may be withholding key information.

Jack Smith said, "We just want Gabriel found.  We want him back here.  That's why we're doing everything humanly possible."

Tammi Smith told INSIDE EDITION's Les Trent she first met Gabriel's mother at an Indianapolis airport on a business trip seven months ago.  She says that out of the blue Elizabeth Johnson asked her to adopt Gabriel. 

LesTrent asked "When you hear her now say that she gave her child to someone she met in a park in San Antonio, do you believe she would have done that?"

Tammi Smith said "I believe that because that's what happened to us."

The Smiths are taking a polygraph test Friday to prove they have no knowledge about baby Gabriel's whereabouts.

"When baby Gabriel does comes home,and we pray that is what happens, he is not coming to our home.  He is coming to his father's home.  That's where he belongs," said Jack Smith.