After the department declared the charges last month, the authorities said that the case against Mr. Riser had begun after Ms. Saenz’s body was found in the Trinity River in Dallas on March 10, 2017. She was shot several times, the authorities said.

In September 2017, three guys — Kevin Kidd, 28; Emmanuel Kilpatrick, 31; and Jermon Simmons, 35 — were detained and charged with killing Ms. Saenz, the authorities said.

On Aug. 12, 2019, a prosecutor told the Dallas police that one of the men — who was identified only as a “ watch ” in court documents — wanted to come forward with information regarding Mr. Riser’s participation in Ms. Saenz’s murder, the police said.

The witness told the police he and Mr. Riser had known each other for years and had reconnected in 2013 when Mr. Riser contacted him and asked him if he was still doing the things they were doing when they were young, like committing burglaries, according to court documents.

The witness said that Mr. Riser had initially promised to supply him with information about drug houses if the witness and his team would rob the homes and keep the drugs and give any stolen money or guns to Mr. Riser, the authorities said.

The strategy, however, never materialized, the witness said, since Mr. Riser then created a plan to have Mr. Douglas kidnapped and murdered for $3,500, the police said.

Several days later, the witness said, he and a member stopped and handcuffed Mr. Douglas, place him in a car and drove him into a place near the Trinity River, the authorities said. Mr. Douglas was captured and killed, and his body was dumped in the river, the witness told the police.