Alabama police have opened a homicide investigation into the death of a 31-year-old Atlanta doctoral student who disappeared during a family 4th of July celebration and was found dead two days later.

Investigators are also looking for Deundray Cottrell’s 31-year-old partner of three years, Julian Taylor Morris, who disappeared the day after Cottrell disappeared.

Cottrell’s body was found Saturday afternoon in east Birmingham, but police have not provided information about how he died, AL.com reported. They did announce on Sunday that his death was ruled a homicide.

Cottrell was last seen at about 10 p.m. Thursday night. His sister said that Cottrell and Morris arrived at their home at about 6 p.m. from Atlanta, where Cottrell worked in marketing and studied toward his doctoral degree. At about 10, Angelica Harris said, he went upstairs to check on his dog because of the fireworks.

“He went and checked on the dog and I never saw him again,’’ Harris said.

Morris was inside the house and told family he spoke with Cottrell who then left running through the backyard.

Family members found his sunglasses, cell phone and a shoe and said Morris was behaving erratically, unlike how they’d known him over the past three years. Eventually, they called police and filed a missing person report.

The family said they continued to search throughout Friday, obtaining security video that showed Cottrell running through yards and jumping over a gate.

Harris said her brother looking like “he was running for his life” in video she saw.

Officers joined the search and found some clothing and another video of Cottrell walking in the area.

Morris, meanwhile, also vanished. Family said they saw him Friday morning but not again after that.

Anyone with information is asked to call homicide detectives at 205-254-7777 or Crime Stoppers at 205-254-7777.