Carlos and Jemima Guimaraes were acquitted of other conspiracy charges in a Houston courtroom.
They were arrested last February while arriving in Miami for vacation and charged with aiding in the kidnapping of their eight-year-old grandson Nico Brann five years ago.
The couple's daughter Marcelle Guimaraes, Nico's mother, allegedly relocated the child to Brazil under false pretenses and without the permission of his American father Chris Brann, a Houston physician.
If the verdict is upheld, the grandparents in their 60s could face up to three years in prison.
But that verdict was thrown into uncertainty after the presiding judge said he would consider the defense's request to override the jury's findings and acquit on all charges, according to the Houston Chronicle.
Such overrides are rarely granted.
The couple presented evidence during trial that the child's mother was fleeing domestic violence, according to local media.
"I and our side are crushed," the couple's lawyer Rusty Hardin said outside the courthouse, according to KHOU-TV.
He testified before the US Congress in 2016 to ask Washington to impose sanctions on Brazil for violating an international standard requiring the return of children to their home country, if they were taken in violation of custody agreements.
A similar dispute led to the Sean Goldman Act in 2014, authorizing Washington to take action in international child custody disputes, ranging from public condemnation of other governments to the suspension of US aid.
Brann claims his ex-wife Marcelle Guimaraes took their son to Brazil in 2013 intending to stay permanently, while telling him the trip would only last a few weeks.
Marcelle Guimaraes's parents allegedly helped her hide her motives, and enrolled the child in a Brazilian school they operated, before the South American country's legal system granted the mother full custody.
In 2016, the parents of seven children illegally taken from the US to Brazil asked then-Secretary of State John Kerry to sanction Brazil under the 2014 law.
They argued Brazil had seen no evidence the US takes such abductions seriously.
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