9-year-old boy was going to a supervised visit when his dad opened fire, police say. Here's how he escaped.

AUSTIN — The boy was only a few days shy of his 10th birthday on the morning that he would see his mom, his half-sister and her boyfriend alive for the last time, as they pulled into a parking lot for a scheduled supervised visit with his estranged father.

Instead of greeting his son with a hug and presents, the father rammed his vehicle into the family's car, pulled a handgun and began to fire without warning, the boy told police, forcing him to flee and seek shelter in a passing driver's car, according to court documents.

Police investigators have accused the boy's father, Stephen Broderick, 41, of shooting to death his wife, Amanda Broderick, 35; his stepdaughter, 17-year-old Alyssa Broderick; and the daughter's boyfriend, 18-year-old Willie Simmons.

As he raced away, the boy flagged down someone who was driving on a nearby road. He scrambled into her car and told her to call 911, and they drove away, an affidavit says. As he gave the witness a frantic recounting of what happened, he told her that his father was mad at his mother about their divorce.

Another woman was there at the time of the shooting, to facilitate supervised visits between the couple and their son, Stephen Broderick's arrest affidavit revealed. Amanda Broderick was supposed to pull into the parking lot on Great Hills Trail in Northwest Austin late that morning and release the boy to the supervisor.

Normally, the supervisor told detectives, she would take the boy inside and then wait for Stephen to arrive 15 minutes later for the supervised visit.

But just as she was about to pull into the parking lot, she heard what sounded like a car crash, followed by gunshots, and she saw two women lying dead on the ground and a man who was lying on the ground and struggling to breathe.

She also saw the man she recognized as Stephen Broderick walking away from the scene, the affidavit says.

Background: Austin shooting suspect got bail decreased, ankle monitor removed months after woman raised concerns

Stephen Broderick, a former property crimes detective with the Travis County sheriff's office, is charged with capital murder and is being held without bail at the Travis County Jail.

He was charged last summer with sexually assaulting a child and fired from the sheriff's office after his arrest.

After he was jailed last year, Broderick posted bail, which was set at $50,000. The courts ordered him not to contact or go within 200 feet of the child whom investigators said he assaulted. Broderick also was ordered to wear a GPS tracking device, but state District Judge Karen Sage later allowed him to remove that monitor.

In an application for a protective order after Broderick's arrest, the woman said she worried about her safety and that of the children.

"I'm afraid he will try to hurt me or (the) children, because these allegations have come out and he may lose his career," she stated. "Stephen has prior military experience and is SWAT trained. If he wanted to hurt someone, he would know how."

More: Former sheriff's deputy Stephen Nicholas Broderick arrested in fatal shootings of 3 in Austin

This article originally appeared on Austin American-Statesman: Austin shooting: How suspect Stephen Broderick's son escaped attack