Robert F. Kennedy Jr., the nation’s health secretary, pledged on Thursday to seek out experts globally to discover the reasons for the increasing rates of autism in the United States.

“We’ve launched a massive testing and research effort that’s going to involve hundreds of scientists from around the world,” Mr. Kennedy announced at a cabinet meeting held by President Trump. “By September, we will know what has caused the autism epidemic and we’ll be able to eliminate those exposures.”

“There will be no bigger news conference than that,” Mr. Trump replied.

But scientists who have worked for decades to find a cause greeted Mr. Kennedy’s predicted timeline with skepticism.

They said that a single answer would be hard to identify in a field of possible contributors including pesticides, air pollution and maternal diabetes.

Dr. Philip Landrigan, a pediatrician and expert on environmental toxins, pointed to the current mass layoffs and cutbacks for research at Mr. Kennedy’s Department of Health and Human Services as one reason for doubting such quick progress.

“Given that a great deal of research on autism and other pediatric diseases in hospitals and medical schools is currently coming to a halt because of federal funding cuts from H.H.S.,” he said, “it is very difficult for me to imagine what profound scientific breakthrough could be achieved between now and September.”

Mr. Kennedy’s office did not offer many details on the plan. Later on Thursday, Mr. Kennedy revealed a few more clues, saying that the National Institutes of Health would lead the effort.

He said the Centers for Disease Control and Prevention would soon release data showing that the autism diagnoses had now increased to one of every 31 children. Many scientists and doctors attributed the rise in autism rates over the last several decades in part to growing awareness of the disorder and to expanded diagnoses along a spectrum.

“We are launching requests to scientists from all over the country and all over the world,” Mr. Kennedy said in an interview on Fox News. “Everything is on the table: our food system, our water, our air, different ways of parenting, all the kind of changes that may have triggered this epidemic.”

In the interview, Mr. Kennedy also said an important part of the effort would be to compare autism rates in vaccinated and unvaccinated children. It’s an angle that many scientists dismiss, saying that parents who vaccinate their children are also more likely to get a diagnosis, given higher rates of interaction with health providers.

Many scientists who have observed Mr. Kennedy’s dogged opposition to many vaccines over two decades — and his frequent attempts to tie autism to vaccines — have dismissed his claims, citing the scientific consensus that debunked any such link. Scrutiny of his pledges to begin “gold standard” research only heightened in recent weeks when David Geier, a largely discredited researcher, was hired at H.H.S. to study any potential link between vaccines and autism.

Irva Hertz-Picciotto, director of environmental epidemiology of neurodevelopment at the University of California at Davis MIND Institute, said she had been seeking a cause for autism for 20 years.

It’s an exciting time for the field, she said, with multiple studies beginning to point to factors that could play a role. Among the possible links are pesticide exposure through food or nearby agricultural fields; workplace exposures to solvents; fevers or infections during pregnancy; maternal diabetes; and exposure to chemicals in plastics called phthalates or to PFAS, known as “forever chemicals” for their tendency to persist in the environment.

Some studies have found that the effect of environmental exposures can vary based on a person’s genes, “emphasizing the complexity of this syndrome.” Dr. Hertz-Picciotto said it was challenging to get funding for clinical studies that could shed light on a cause of autism, but new research could help advance the field. Those studies, though, could take years.

That all of the research could be sorted out by September, she said, was “ludicrous.”

Dr. Alice Kuo, the chief of pediatric medicine who cares for children and adults with autism at the University of California, Los Angeles, said she was involved in a yearslong project by the National Institutes of Health called the National Children’s Study. It followed thousands of children and parents and elicited information, including the possible causes of autism. The study was costly and ended far earlier than planned, Dr. Kuo said.

She said an effort to do something as rigorous would take months to plan and design — and years to arrive at an answer.

In a social media post on Thursday, Mr. Kennedy recognized the assistance of the president and the Make America Healthy Again Commission — a subgroup of the cabinet that includes Linda McMahon, the education secretary, and Russell Vought, director of the Office of Management and Budget. Like Mr. Kennedy, they are not doctors or scientists.

“Thanks to your MAHA Commission, we’ll soon identify the root causes of the autism epidemic,” Mr. Kennedy wrote in a social media post.

Created by an executive order in February, the commission is charged with assessing in 100 days the threat to children of food ingredients, chemicals, medications and other exposures. Within 180 days, the commission is expected to submit a strategy to the president to address its findings.