(CNN) -- San Francisco on Wednesday threatened to sue Nevada over the practice of "patient dumping," in which the state allegedly sent hundreds of indigent mentally ill patients on one-way trips to California.
In a letter to Nevada's state attorney general, San Francisco City Attorney Dennis Herrera threatened a class-action lawsuit, saying Nevada's state-run psychiatric hospital Rawson-Neal sent two dozen patients to his city and hundreds more to other spots in California without any arrangements for care once they arrived.
"As part of my office's investigation, we have obtained the names of the almost 500 patients whom Rawson-Neal discharged and sent by Greyhound bus to California since April 2008," Herrera says in his letter to Nevada Attorney General Catherine Cortez Masto.
Herrera said San Francisco has spent at least $500,000 on medical care, housing and subsistence grants for the patients since they arrived from Rawson-Neal.
A spokeswoman for the Nevada attorney general's office, Jennifer Lopez, said the office received the letter and was working on the matter.
"We cannot comment further on pending litigation," she said.
The Sacramento Bee newspaper first reported on the practice in March when it told the story of former Rawson-Neal patient James Flavy Coy Brown, 48, who was sent on a 15-hour bus ride to Sacramento despite knowing no one in the area, never having visited, and having no arrangements for his care, housing, or medical treatment.
Rawson-Neal put Brown in a taxi to the Greyhound bus station with a one-way ticket, snacks, and a three-day supply of medication to treat his schizophrenia, depression and anxiety, the paper reported.
Brown said he was told to call 911 when he arrived. He was "completely confused" with no idea why he was sent to Sacramento, a social services worker told the Bee. Brown disappeared in Sacramento, according to the newspaper, and his whereabouts remain unknown.
In April, after reports that Rawson-Neal bused more than 1,500 psychiatric patients to locations across the United States, Herrera's office launched an investigation to find out whether any of the 31 Greyhound bus tickets the hospital bought for trips to San Francisco were for improperly discharged patients, a statement from the city attorney said.
The investigation discovered 24 patients were sent to San Francisco, including some who made multiple trips, and that 20 sought and received emergency medical care once they arrived, Herrera's office said.
That was just San Francisco, however. Herrera's office said it later found that 500 Rawson-Neal patients were sent to locations throughout California.
The alleged busing of patients "unlawfully burdened California local government resources ... in order to provide care and services which Nevada was legally obligated to provide to its own indigent residents," Herrera's office said in a statement.
"While our prospective litigation asserts the rights of California taxpayers, it's equally necessary to protect society's most vulnerable from continued institutional abuse," Herrera said. "Homeless psychiatric patients are particularly defenseless from the kind of lawless 'patient dumping' practices Nevada officials engaged in."
Herrera said Nevada can avoid a lawsuit if it reimburses California local governments for the cost of taking care of the patients and adopts binding protocols to end the busing practice, his office said. Nevada must respond to the offer by September 9.
CNN's Mayra Cuevas contributed to this report.