RIO DE JANEIRO — The number of fires in the Amazon rainforest dropped significantly in September, a month when fires typically increase, Brazil’s satellite research agency reported.

The number of fires since the beginning of the year continued to outpace 2018, according to data watched closely by scientists and activists.

Around 20,000 fires burned in the Brazilian portion of the Amazon during September, Brazil’s National Institute for Space Research reported. That was a 35 percent drop from August, when roughly 31,000 fires burned — a nine-year high. It was nearly a 20 percent drop from September 2018, when 24,500 burned.

The plunge — which followed a summer of international attention on the fires, and the Brazilian government’s response to them — caught Amazon watchers by surprise.

Since measuring began two decades ago, September, the peak of dry season, has almost always been more flammable than August. That history fueled expectations that the problem would only worsen as the dry season deepened.

Analysts said rain and the government response — hundreds of soldiers were dispatched to fight the fires — might have been factors.

“It will take some time to determine the impact of government interventions,” said Daniel Nepstad, the executive director of the Earth Innovation Institute in San Francisco.

The overall number of fires in the Amazon biome from January through September was nonetheless 43 percent higher than the same period last year, according to the institute.

The increase in fires ignited an extraordinary summer in Brazil. Smoke blanketed faraway cities. A viral campaign broadcast images of the forest in flames. International leaders and celebrities criticized the response of President Jair Bolsonaro.

Bolsonaro ran for office on promises to promote development in the Amazon. He is accused of easing environmental protections, which critics say has encouraged illegal deforestation.

As the fires increased, he accused his critics of lighting them to make him look bad. He rejected a multimillion-dollar aid package offered by the Group of Seven as an assault on Brazil’s sovereignty.

Fire is a key tool in the process of deforestation. Analysts blame most of it on land grabbers burning the forest to clear land for cattle and soy farms.

Fire is used when the forest is too thick or too remote to knock down with bulldozers. It can also sow nutrients into the ground that can help boost agricultural production.

The September drop remained a mystery.

“It is not simple to find out the reasons for the decline in fires while deforestation continues to increase,” said Carlos Nobre, a Brazilian scientist. “The army was sent to combat fires, and not necessarily to combat the environmental criminals doing the deforestation. It is obviously simple to detect a fire due to the smoke plumes, and the Army and police can act readily against it.”

Many of the fires are small — but in a forest ill-equipped to withstand any flame, they can wreak devastating damage.

Scientists worry that increasing rates of deforestation and fire could push the Amazon to a tipping point, at which the ecosystem is knocked off balance and large swaths convert into an arid savanna.