The Dayton mass shooting gunman drove to the scene of the carnage with his sister — then made her one of the nine victims killed in the slaughter, officials revealed Sunday.

Megan Betts, 22, was shot dead when her brother, 24-year-old Connor Betts, opened fire on a crowded street in the Ohio city lined with bars and restaurants around 1 a.m., officials said in a press briefing.

But before the sororicide, the siblings and a mutual acquaintance drove together to the hopping nightlife district in Betts’ father’s 2007 Toyota Corolla, officials told reporters, piecing together what they had learned in the hours since the bloodshed.

Sometime after the trio arrived, Connor Betts split up from the other two, investigators said — during which time he grabbed his AR-15-style rifle, crept through an alley towards the row of revelers and opened fire.

Firing the rifle, equipped with a double-drum magazine with a 100-round capacity, Betts gunned down the first of his victims near a tattoo parlor, then moved across the street towards Ned Peppers Bar, officials said.

Responding cops, including at least two armed with long guns, fatally shot Betts within 30 seconds of his first shot, officials said — but that was all the time he need to kill nine people and wound 27 more.

Megan Betts was among the dead, and the siblings’ companion was wounded, according to authorities.

The other eight deceased victims were identified by officials as Lois Oglesby, 27; Saeed Saleh, 38; Derrick Fudge, 57; Logan Turner, 30; Nicholas Cumer, 25; Thomas McNichols, 25; Beatrice Warren-Curtis, 36; and Monica Brickhouse, 39.

Both the rifle and a shotgun recovered from inside the Toyota were legally purchased by Betts, said Dayton Police Chief Richard Biehl.

Investigators have yet to publicly identify a specific motive in the shooting — and said it’s unclear whether Megan Betts was her brother’s intended target.

“It’s a nagging question and I just don’t have the answer,” said Biehl, adding of the Betts family, “This is a nightmare for them, and I think they are struggling, as you can understand.”