The two people arrested in the killing of Reserve couple Lakeitha and Kenneth Joseph were a couple who met across state prison bars, court documents in the case show.

One of them, Horatio Johnson, 37, is also Kenneth Joseph’s cousin, according to a New Orleans police detective’s application for an arrest warrant.

Johnson was serving a 35-year prison sentence for manslaughter in the fatal shooting of a LaPlace man in 1993, admittedly over a drug deal. Brittany Martin, 25, was a state prison guard at the same facility, according to NOPD Detective Ryan Vaught.

“She would later resign under investigation for engaging in an improper relationship with Horatio Johnson while he was an inmate at the facility,” Vaught wrote.

A state Corrections Department spokeswoman did not respond Tuesday to questions about the reason or date of Martin’s resignation.

The June 6 arrest warrant came a day before Johnson and Martin turned themselves in to U.S. marshals over a double-killing that ended with a seemingly preplanned body dump into the Intracoastal Waterway in New Orleans East.

The pair was booked Friday into the Orleans Parish jail, where bond was set at $1 million for Johnson and $800,000 for Martin, each facing two counts of second-degree murder. Both live in LaPlace.

The application from Vaught adds some new details to the apparent attempt to get rid of evidence from a crime that took place sometime after Feb. 18, when the Josephs disappeared after Kenneth Joseph borrowed a 2010 Dodge Caravan from his sister.

Vaught wrote that another man, identified only as “Witness #1,” told police that he, Johnson, Martin and a man named “Blue” delivered the van to Frank Mike Jr. to clean it up and get rid of it. The witness said blood was on the van’s passenger-side floor.

New Orleans Police Superintendent Ronal Serpas has declined to say whether detectives believe the Josephs were alive or dead at the time they were dumped into the water. The Orleans Parish Coroner’s Office has classified the deaths as homicides by drowning.

In a news conference last week, Serpas and St. John the Baptist Parish Sheriff Mike Tregre said they continue to pursue others in connection with the crimes.

Mike initially told investigators he found the Caravan in the driveway of a home he had been renovating in New Orleans.

According to the arrest warrant application, Mike later told police he received the van from Witness #1, Horatio Johnson and “Blue,” with the witness telling him, “We pulled off a little move in the van.”

He later dumped the van in Georgia after he saw a news story that linked it to the missing persons case.

A federal grand jury on April 25 handed up an indictment charging Mike with knowingly transporting a stolen vehicle across state lines. He is accused of taking the Caravan to Georgia after the murders, attempting to wipe it free of fingerprints and abandoning it.

The arrest warrant says a call to Crimestoppers tipped St. John the Baptist Parish sheriff’s detectives that the person responsible for the couple’s disappearance was “an unnamed cousin of Kenneth Joseph” who had committed a homicide in Reserve and was later released after serving 18 years in prison.

Johnson was released from state custody in 2011 after serving about half of a 35-year sentence on a manslaughter conviction for gunning down Keith Selders in a schoolyard in 1993. Selders was 30 at the time. Johnson was 17.

Investigators found that Kenneth Joseph and Johnson were cousins, Vaught wrote.

Serpas declined last week to describe a motive for the killings.

The couple’s bodies were found separately in the water, bound with blue rope, weeks after they were last seen Feb. 18.

Fishermen spotted Kenneth Joseph’s corpse on March 22, nearly two weeks after his wife’s body turned up. He was tethered to the handle of a 30-pound kettlebell, and detectives believe a similar weight was used on his wife but came loose.

Vaught found that Martin had used a credit card Feb. 19 at a Wal-Mart store to buy two kettlebells, two fleece hooded sweatshirts, three pairs of “gripping gloves,” fishing line, blue utility rope, two pairs of shoes and a bottle of degreaser, court records show.

The arrest warrant application does not say how Vaught knew to go to Wal-Mart, only that he “located a retail corporation in the metro area where he believed the murder weapons, a kettlebell and blue rope, may have been purchased.”

Martin is due back in court June 9. Johnson has a preliminary hearing scheduled for May 28.

Follow John Simerman on Twitter, @johnsimerman