A man suspected of slaughtering five people, including his parents and girlfriend, was arrested Sunday following a day-long manhunt when he showed up at his grandmother’s house in Virginia, authorities said.

Cop say Dakota Theriot, 21, began his killing spree Saturday morning, gunning down his girlfriend, along with her brother and father, in their Louisiana home, before driving 30 miles and shooting his parents dead in their trailer.

The suspect’s father, Keith Theriot, 50, was still alive when authorities arrived and was able give “a dying declaration … that it was his son that committed this act,” said Ascension Parish Sheriff Bobby Webre.

Neither Keith nor his wife, Elizabeth, 50, survived.

Their son then drove 16 hours and more than 1,000 miles in a Dodge pickup truck he stole from his girlfriend’s dad, hoping to find a safe place to hide with relatives in Virginia, authorities said.

Though Theriot didn’t reach out to his grandmother, she was so scared he might try to find her that she stayed in a hotel Saturday night, officials said.

Richmond County sheriff’s deputies went to check on the home Sunday morning — and Theriot pulled into the driveway.

He had a gun on him, but he dropped it and was taken into custody, Sheriff Stephan Smith said.

Theriot will face five counts of first-degree murder, among other charges, authorities said.

A couple of weeks ago, his parents had asked him to move out, so he shacked up with his 20-year-old girlfriend, Summer Earnest, and her family, including dad Billy, 43, and younger brother Tanner, 17, authorities said.

The pair had just started dating.

Authorities still don’t have a motive for the murders.

With Post wires