•  Though Sierra had a lengthy arrest record that included drug and domestic violence charges — and had the two biting incidents on his resume — he was allowed to move right back in with his wife and her three youngsters after leaving jail this summer.

Walter Lambert, who heads the Miami-Dade Child Protection Team and examined Julia a year ago, declined to discuss specifics of the case. In general terms, however, he said he has long viewed bite injuries as one of the most dangerous red flags in child welfare.

“People who bite will also break bones,” said Lambert, a pediatrician who specializes in evaluating children for signs of abuse and neglect. “And they will also kill children.”

The teeth marks on Julia’s arms first were detected by teachers and administrators at Miami’s Seminole Elementary. Julia told DCF that her stepfather bit her because she was going to take her younger sister’s bottle. Julia said her stepfather was “being mean.”

However, Julia’s mother gave investigators another story altogether: Her husband, she said, was only “playing” when he bit the girl. “I overheard from Julia that Papy is biting me,” she told DCF. She said Julia “was laughing at the same time.”

The Child Protection Team concluded the bite mark was the result of “child physical abuse.”

But by the time DCF closed the case 20 days later, agency workers had decided to do nothing.

Machado, DCF records show, had vowed to divorce Sierra: “He must have received the divorce papers already,” she said. Sierra, a convicted felon with a gun in the house, was headed to the Miami-Dade County Jail for violating probation. And Padrino, the children’s birth father, was living in Machado’s home “temporarily” to help her care for the children.

“The children are safe,” a Dec. 20 DCF report says. “The alleged perpetrator, Alberto Sierra, is currently incarcerated.”

Maj. Connie Shingledecker, who oversees the Manatee County Sheriff’s Office child protection unit, and has long been a member of the Statewide Child Abuse Death Review Committee, said child welfare authorities confront a vexing problem when mothers who endure domestic violence are unwilling or unable to part with their husbands or boyfriends. Child abuse investigators are reluctant to remove children solely because their mothers are being battered. But men who beat their wives are likely to abuse their children, as well, so child welfare workers often strongly encourage mothers to leave their abusers — even threatening to remove their kids if they do not.

“Especially with really young children, we can protect them if their mother is unwilling,” Shingledecker said.

In all, DCF had made contact with Machado four times: a 2010 allegation of substance abuse, inadequate supervision and physical injury, another 2010 drug allegation, and a 2011 domestic violence allegation — all closed as unverified. Then there was the October 2011 case that began with Julia’s bitten arm, which was verified as physical abuse. The 2010 drug use, poor supervision and physical injury investigation has not been released to The Herald by DCF.

When DCF investigators evaluated the safety of Machado’s children in the fall of 2011, the available record for Alberto Sierra consisted of the following: Since 2000, he had been arrested 15 times, including multiple marijuana and cocaine charges, drug trafficking, aggravated assault, soliciting a prostitute, grand theft, weapons possession and domestic violence.