Florida Attorney General James Uthmeier said during a press conference the company suppressed internal safety warnings and deceived users about the true nature and dangers of the product.
He said Florida was the first state to sue OpenAI.
"Today, we announced the first-in-the-nation state-led lawsuit against OpenAI and its CEO, Sam Altman," Uthmeier said. "OpenAI and Altman ignored internal and external safety warnings, put children at great risk, and allowed a dangerous product to reach millions of Floridians."
The lawsuit filed in Florida circuit court references two separate shootings where the alleged gunmen were reported to have asked ChatGPT questions while planning their crimes.
OpenAI said in a statement its models repeatedly encouraged the individuals to seek real-world support, including from mental health professionals. The company also said it has co-operated with law enforcement in both cases.
"ChatGPT is a general-purpose tool used by hundreds of millions of people every day for legitimate purposes," an OpenAI statement said. "We work continuously to strengthen our safeguards to detect harmful intent, limit misuse, and respond appropriately when safety risks arise."
In April, Uthmeier opened a criminal investigation into OpenAI over whether ChatGPT offered advice to a gunman who killed two people and wounded six others last year at Florida State University.
In another case, prosecutors have said the man charged with killing two University of South Florida doctoral students had asked ChatGPT what would happen if a human body was put in a garbage bag and thrown in a dumpster, days before they went missing.
Florida's lawsuit alleges OpenAI and Altman prioritised speed to market and commercial gain over user safety and disregarded repeated warnings from experts both inside and outside the company.
The lawsuit claims the company deployed a product that facilitates and encourages harm, including self-harm and violence, while falsely assuring users it was safe.
