A North Texas home health care worker, already charged with murdering an elderly woman last year, is now facing 11 more murder charges as well as several attempted murder counts.

Billy Chermirmir, 46, first prompted an investigation into hundreds of deaths of senior patients across North Texas after he was charged with killing Lu Thi Harris, 81, in Dallas County last March. The women were all killed after he allegedly smothered them with a pillow, according to Dallas ABC affiliate WFAA. Some of the murders go back for several years.

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This week, ABC News reported that Chemirmir was indicted on six additional murder charges in Dallas County. The home health aid is now charged with killing Phyllis Payne, 91, on May 14, 2016; Phoebe Perry, 94, on June 5, 2016; Norma French, 85, on Oct. 8, 2016; Doris Gleason, 92, on Oct. 29, 2016; Rosemary Curtis, 76, on Jan. 17, 2018; and Mary Brooks on Jan. 31, 2018.

He was also charged on Tuesday with five more murders in Collin County, which is a suburb of Dallas, according to jail records. No additional details for those murders were released. Chemirmir also faces charges for three attempted murders in Dallas County and two attempted murders in Collin County, authorities said.

Chemirmir is being held in Dallas County jail on a bond of $9.1 million. Because he is a citizen of Kenya, he also has an immigration hold filed against him in Dallas County, ABC News reported.

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After Dallas police arrested Chemirmir on charges of murdering Harris last year, David Pughes, Dallas Police Department Executive Assistant Chief of Police said police would examine about 750 similar cases to see if Chemirmir could have been involved.

“It will be a monumental task,” Pughes said at a press conference at the time. “But we’re up for the challenge and we’re gonna make sure we check each and every case.”

Chemirmir was first arrested last March for an outstanding warrant. But police said they observed him ditching a jewelry box into a trash dumpster in Plano, and were able to trace the jewelry to Harris’ home. Once inside her home, police found the woman dead from being smothered.