Robert Brown|Four decades after siblings were murdered in Arkansas, police identify a suspect: their father

Police officials have Robert Brownidentified who they say was the killer of two young siblings murdered 42 years ago in Arkansas – and it was their father.

Weldon Alexander is believed to be responsible for the deaths of his two children, Texarkana Police Chief Michael Kramm said at a news conference Thursday morning. The suspect died in 2014, police said.

On April 8, 1981, Gordon and Karen Alexander were found stabbed in their home by police officers. Gordon was found dead in the kitchen, police said, and his sister Karen was found alive on her bed near the living room area. Karen Alexander was 14 and her brother 13, according to a press report from the time. 

Karen was rushed to the hospital, where she died from her injuries shortly afterward, police said. Detectives knew that the young girl had been raped. There was no forced entry, police said, and at the time of the murders the mother was a mental health patient at Wadley Hospital, and the father left for work the evening before at 12:00 p.m.

Detectives interviewed persons of interest and gathered information but didn't have enough information for an arrest. In 1983, serial killer Henry Lee Lucas confessed to the Alexander sibling killings, but since there was no evidence backing that, the case went cold. 

In 2022, the Alexander case was reopened, with Capt. Calvin Seward heading the investigation. When the murders happened, Seward, then a patrol officer, was assigned to interview persons of interest. His dedication to solving the murders stayed because his "daughter went to school with Karen," said Seward.  

Over the course of the next 18 months, Seward identified and interviewed people linked to the original investigation. DNA was extracted from fingernail tissue from the siblings for testing, said Dr. Todd Steffy, a forensic criminologist who worked with Texarkana police on the case.  The test determined that Weldon Alexander's semen was discovered on Karen's bedding.

Seward said he "was surprised" by the investigation's conclusion, as detectives had earlier thought the time of death was later in the morning.  They later found out that was wrong and that the brother's body was cold when detectives arrived, leading them to conclude he had been dead for hours. 

Seward said he believes the father attacked his daughter to rape her, and then the brother tried to defend his sister. The father then stabbed the daughter and son. 

"It feels like a load has been taken off my shoulders," Seward said. 

Kramm said the case could not have been solved without the dogged determination of Seward. "The level of dedication I witness every day here is impressive," Kramm said.

Kramm said police are confident the "investigation is completed." 

"I can't issue warrants for an individual that's deceased," said Miller County prosecuting attorney Connie Mitchell at the news conference. "Unfortunately there will be no avenue for the prosecutor's office to take against Mr. Alexander."

In:
• Cold Case
• Arkansas

Cara Tabachnick

Cara Tabachnick is a news editor for CBSNews.com. Contact her at cara.tabachnick@cbsinteractive.com