Federal and state authorities announced steps this week to rein in for-profit stem cell clinics from New York to Florida, ramping up efforts to rein in a booming industry that critics say is offering unapproved and expensive treatments that have harmed a growing number of patients.

New York state’s attorney general filed a lawsuit Thursday against a Manhattan clinic called Park Avenue Stem Cell, accusing it of “fraudulent and illegal advertising regarding its stem cell procedures.”

A day earlier, the Food and Drug Administration sent 20 letters to stem cell companies, telling them to stop selling unapproved products in violation of federal laws.

The FDA also sent a more formal letter of warning to an Altamonte Springs, Fla., company called Cord for Life, that has sold umbilical-cord-blood products to stem cell companies, accusing it of legal and safety violations.

The actions are part of a growing attempt by government officials to regulate a burgeoning industry that critics say has injured at least two dozen patients in recent years and cheated thousands more. Hundreds of clinics have popped up selling stem cell procedures — not covered by insurance, unproven by science and unauthorized by the government — that the clinics claim can treat ailments ranging from creaky knees to Alzheimer’s, Crohn’s disease and multiple sclerosis.

Such clinics are “misleading vulnerable consumers who are desperate to find a treatment for serious and painful medical conditions,” New York Attorney General Letitia James (D) said in a statement, calling such behavior “unacceptable, unlawful, and immoral.”

The New York lawsuit — seeking an injunction to stop Park Avenue Stem Cell from marketing its treatments — is one of the first instances in which a state has filed a lawsuit against a stem cell company.

The lawsuit accuses Park Avenue Stem Cell of misleading patients by telling them without proof that its treatment can treat conditions such as erectile dysfunction, neurological diseases and cardiac problems. The treatments start at $4,000, and some patients paid for multiple sessions, state authorities said.

Park Avenue Stem Cell did not respond to calls or emails for comment.

At the federal level, FDA Commissioner Scott Gottlieb has stepped up threats against clinics, asserting their products are illegal drugs and vowing to take stronger enforcement action against them, such as the letters sent Wednesday.

Scientists say stem cell research may someday lead to solutions to complex and in­trac­table diseases. But scientific societies, medical associations and leading researchers have issued warnings about for-profit clinics and compared them to snake-oil salesmen.

At least five patients have suffered severe vision loss after having treatments injected into their eyes. This year, a Washington Post report documented at least 17 people hospitalized with severe bacterial infections after being injected with stem cell products made from umbilical-cord blood, a little-known but fast-growing segment of the industry.

Some critics question whether Gottlieb’s efforts have had much effect.

“It’s not that these letters are inconsequential,” said Leigh Turner, a University of Minnesota bioethicist who has documented the industry’s growth. “But if you’re trying to tackle this one business at a time, you’re not going to make a dent. We’re talking about one new warning letter when there are now hundreds of clinics out there. . . . Why not send out 50 warning letters, 100 warning letters?”

FDA officials said the agency has sent out five warning letters in the past two years, including Wednesday’s.

In an interview, Gottlieb said that the FDA was slow in the industry’s early years to respond but contended it has picked up the pace.

“I do think warning letters have had a deterring effect,” he said, adding that each warning letter requires immense FDA resources.

“We can’t just write warning letters like speeding tickets,” he said, noting that each requires inspections and intense documentation so that if the company ignores the letter, the government can sue them. But in recent years, such lawsuits have been rare, with the government going after only a handful of stem cell companies in court.

Gottlieb recently resigned and is set to leave office Friday. Some stem cell experts said they remain hopeful Gottlieb’s efforts may have an effect.

“The unmistakable message is that the firms’ time to come into compliance is rapidly running out,” said Paul Knoepfler, a stem cell biologist at the University of California at Davis and frequent critic of the for-profit stem cell industry. “It may be by the end of 2020 we’ll see fewer risky stem cell clinics out there than in 2019.”

The FDA’s new warning letter to Cord for Life, a Florida-based cord-banking company, cites the company for safety lapses, such as not following specific sterility procedures or properly documenting its tests and equipment.

Cord for Life general manager Donald Hudspeth said he believes his company — which he said has operated as a cord-blood bank for 25 years without major compliance issues — came under federal scrutiny because of its brief involvement with Liveyon, a company whose product derived from umbilical cord blood led to the hospitalization of 15 patients last year with severe bacterial infections. Those infections happened before Liveyon started using Cord for Life as a supplier.

Liveyon said it had obtained the product from a San Diego manufacturer called Genetech. After the infections, Liveyon officials said they stopped using Genetech and briefly used Cord for Life for a few months.

In a public version of the warning letter sent to Cord for Life, FDA officials redacted the name of a third party copied on the letter. But the unredacted version shows Liveyon as the third party, according to a person who had seen the letter.

Hudspeth said his company has already taken action to address the safety issues raised by the FDA.