A homeless man was charged Tuesday in the killing of a Los Angeles emergency room nurse who was attacked last week near the city's Union Station.

Sandra Shells, 70, suffered a fractured skull in the attack on Thursday and police said they located Kerry Bell, the 48-year-old suspect, not far from the scene.

"Sandra Shells unselfishly dedicated her life to caring for others in their time of need," George Gascón, the city's district attorney, said in a news release. "To lose such a valuable member of our community is tragic, and my office will hold accountable the person responsible for her death."

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People wait in line to get the COVID-19 test at Union Station on Monday, Dec. 27, 2021. (Myung J. Chun / Los Angeles Times via Getty Images)

People wait in line to get the COVID-19 test at Union Station on Monday, Dec. 27, 2021. (Myung J. Chun / Los Angeles Times via Getty Images) (Myung J. Chun / Los Angeles Times via Getty Images)

The attack occurred at a bus stop shortly after 5 a.m., as Shells was on her way back to work at Los Angeles County-USC Medical Center, the release added. Bell allegedly punched Shells with such force that she fell backward and hit her head on the ground, FOX 11 Los Angeles reported.

Shells was taken to the hospital where she worked and died three days later. 

Members of the LAPD are seen delivered a floral arrangement to Los Angeles County-USC Medical Center staff in memory of Sandra Shells, according to LAPD Captain Elaine Morales.

Members of the LAPD are seen delivered a floral arrangement to Los Angeles County-USC Medical Center staff in memory of Sandra Shells, according to LAPD Captain Elaine Morales. (LAPD)

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Following the attack, Metro officials vowed to improve safety on the transit system.

Los Angeles County Supervisor Hilda Solis called for an enhanced safety plan for Metro riders and staff, "so we can prevent tragic incidents like this from happening again."

Meanwhile, County Supervisor Kathryn Barger, who is also on the board of the county Metropolitan Transportation Authority, urged the county to work with public safety, mental health, and transportation providers to create a safer environment. 

"Our essential workers are battling a pandemic and working long hours in tough conditions — at a minimum, they deserve that," Barger added. 

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The hospital remembered Shells as a tireless worker who will "always be remembered as a ‘kind, compassionate and giving nurse."

The case remains under investigation, authorities said. 

Fox News' Edmund DeMarche and the Associated Press contributed to this report