GALVESTON, Texas -- The possible mother of a baby found dumped on the side of a Galveston road has become less cooperative, officials told KPRC Local 2 Thursday.

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Slideshow: Baby Boy's Body Found In Galveston

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An autopsy performed Wednesday determined the cause of the 3-month-old's death was homicide by massive head trauma.

Police believe Travis "T.J." Mullis is the boy's father. Investigators said they have not been able to locate him and have expanded their hunt for him to be nationwide.

The woman who could be the mother has identified the deceased baby in a photo as her son, police said. She got in touch with police after she called hospitals looking for her son. Investigators said she cooperated with their investigation at the beginning but has since hired an attorney.

"She was still cooperating up until recently," Lt. Jorge Trevino said. "One of the investigators told me that she's not as cooperative."

Investigators said the woman and Mullis argued the night before the baby's body was found. Police said they did not know if the argument played a role in the baby's death, if he is determined to be their child.

Mullis left with his son sometime between 5 a.m. and 5:30 a.m. Tuesday to get a car repaired, and the couple was no longer arguing, Trevino said.

A couple discovered what is believed to be the body of Mullis' son and a car seat near a utility pole along Seawall Boulevard near Ferry Road about four hours later.

Mullis was last seen driving a silver, four-door 2002 Hyundai Accent with Texas tag 067-RJP.

Sgt. Jeremy Schwartz said, "The only thing that we have specific about it is it has some duct tape on the right rear passenger door handle."

Investigators said Mullis might have dyed his hair blond before his disappearance. He has been known to spend time in Baytown and Galveston, officials said.

Police said they have several leads as to Mullis' whereabouts. They spent part of Wednesday night tracking down a lead in Alvin, but it did not pan out. Investigators have also checked out leads throughout the Houston area.

The search for Mullis has expanded to the entire United States. Detectives said they hoped national media coverage of the case would help located him quickly.

Police said the FBI has offered its help in the case, but has not yet become involved.

Detectives said they do not know of any credit cards or cellular phones Mullis has used since his disappearance.

Investigators said a Houston hospital faxed them the footprints of Mullis' child, but the quality was too poor to make a positive identification. Officers planned to go to the hospital in person to obtain the originals.

Anyone with information on Mullis' whereabouts was asked to call Galveston police at 409-765-3662.

The baby is the second child to be found dead in the area in the past few months. A fisherman discovered the body of a young girl in a plastic box floating in Galveston Bay in October. Detectives named her "Baby Grace" until they learned her true identity, Riley Ann Sawyers.

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Slideshow: Remembering Riley Ann Sawyers

Slideshow Of 'Baby Grace'

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Riley's mother and stepfather have been charged with capital murder in connection with her death.