Kaufman, Texas (CNN) -- Fears that a white supremacist gang or someone else is targeting Texas law enforcement officials spread Monday to Houston, where the chief prosecutor went under 24-hour protection in the wake of the weekend shooting death of his counterpart in a suburban Dallas county. Kaufman County -- where District Attorney Mike McLelland and his wife, Cynthia, died Saturday -- and Harris County were among numerous Texas and federal jurisdictions that participated in a task force targeting the Aryan Brotherhood of Texas in 2012. The investigation resulted in an indictment that a federal prosecutor called a "devastating blow" to an organization investigators say is known to use threats and violence against its enemies. The McLellands were found shot to death in their house. McLelland's death was the second killing of a Kaufman prosecutor since January 31, when Assistant District Attorney Mark Hasse died in a shooting. At the time, McLelland made a promise to the killer to "pull you out of whatever hole you're in." While authorities have not said if they have established a link between the deaths of Hasse and McLelland, or the involvement of white supremacists, Texas law enforcement agencies did warn shortly after the November 2012 indictment that there was "credible information" that members of the Aryan Brotherhood were planning to retaliate" for the indictment. 'On heightened alert' In Harris County, which includes Houston, Sheriff Adrian Garcia put District Attorney Mike Anderson and his family under 24-hour security in the wake of McLelland's death, said Sara Marie Kinney, a spokeswoman for the prosecutor's office. Uniformed officers have also been placed outside Anderson's office, she said. In Kaufman County, the district's attorney's office will remain closed Monday, but the courthouse will reopen under heavy security. Judges and others are also following law enforcement advice on personal security, county Judge Bruce Wood said. "We are all on heightened alert. There's no question about that," Wood said on CNN's "Starting Point" on Monday, a few hours before the county courthouse was scheduled to reopen. Investigators at the McLellands' home recovered several shell casings from a .223-caliber rifle, a law enforcement source said Sunday. Authorities from numerous law enforcement agencies, including the Texas Rangers and the FBI, are investigating the deaths. While authorities have not offered a motive, speculation quickly fell on white supremacists, whom McLelland had said could be involved in Hasse's death. He died after being gunned down in broad daylight outside the county courthouse. The killing remains unsolved.