St. Paul police arrested a 33-year-old woman who allegedly dragged her 11-year-old son from his bunk bed Monday morning and tossed him over the fourth-floor apartment balcony, seriously injuring the child.

A motive for the attack remains unclear.

A resident at the St. Anthony Park neighborhood apartment complex on Hampden Avenue called 911 around 8:40 a.m. reporting an attempted suicide after witnessing what the resident thought was a child jumping off an upper floor balcony, said police spokesman Steve Linders.

Responding officers found the boy, who was conscious and breathing, suffering from traumatic injuries in the building's courtyard. "He's not communicating with me," an officer relayed to dispatch, according to audio transmissions. "It looks like he might have some broken legs."

Paramedics scooped him up and sped toward Children's Minnesota, but were quickly redirected to Regions Hospital due to the nature of his injuries. The child, who is expected to survive, was treated for at least one broken femur, a fractured jaw and head injuries, Linders said.

Investigators later concluded that his mother had yanked him from bed then dragged him to the balcony and heaved him over the railing, police say. A 6-year-old sibling in the home appeared to be physically unharmed.

"It makes no sense," Linders said of the incident, adding that it deeply affected first responders. "These are some of the most difficult calls that officers respond to — because many are parents themselves."

Chief Todd Axtell called all 11 officers on the scene this morning to offer his support and thank them for doing everything in their power to comfort the child.

"As the grandfather of seven little ones and someone who has responded to calls like this — calls involving the unthinkable — I know the emotional toll this can take on a person," Axtell wrote on Facebook. "Stories like this remind me of just how fragile and precious life is."

The St. Paul woman is being held on suspicion of aggravated assault and malicious punishment of a child. The Star Tribune typically does not name a suspect before they've been charged in court. The mother's girlfriend was also brought in for questioning.

The suspect has no criminal record in Minnesota beyond some traffic violations.

A spokeswoman representing Union Flats, the apartment complex where the fall occurred, released a statement to the Star Tribune.

"Our first and foremost concern is for the child involved in this tragic incident which is our highest priority," she wrote.