Hundreds of law enforcement officers were scouring the railways, creeks and roads in a suburban county near Philadelphia on Friday, looking for a man who escaped from prison a little over two weeks after being convicted of first-degree murder.

The authorities said on Friday that local, state and federal officials were using “helicopters, drones and dogs” to find the convicted murderer, Danelo Cavalcante, who escaped from Chester County Prison early Thursday. He was last seen a little before 10 a.m. that morning by someone going to work at the prison, walking south and wearing a white T-shirt, gray shorts and white sneakers.

“We believe that he is still in the general area,” said Deborah Ryan, the Chester County district attorney, at the Friday news conference. “We believe that he is hiding somewhere locally and that he is alone.”

Ms. Ryan said that people in the area needed to be on “high alert” as Labor Day weekend commenced, suggesting that they lock their doors and keep watch over their children. “We have every reason to believe he is considered dangerous at this time,” she said.

West Chester University, a public college about six miles from the prison, canceled classes for the day, citing “an abundance of caution.”

Mr. Cavalcante, 34, was sentenced to life in prison on Aug. 22. Less than a week earlier, a jury found him guilty of stabbing his ex-girlfriend, Deborah Brandao, to death “in front of her two young children,” Ms. Ryan said in a Facebook post last month.

After killing Ms. Brandao, in April 2021, Mr. Cavalcante tried to make his way back to his home country of Brazil, but was arrested by the police in Virginia, the district attorney said at the news conference. Ms. Ryan suggested that he might be heading south again. He was described as five feet tall and someone who speaks Spanish and Portuguese.

According to the district attorney’s office, investigators learned after the arrest that Mr. Cavalcante was wanted for murder in Brazil. They concluded that he killed Ms. Brandao because she had threatened to tell the police.

In a statement released on Friday, Brazilian law enforcement authorities said that Mr. Cavalcante was charged with shooting and killing a man in November 2017, apparently in a dispute over “an alleged debt related to the repair of a vehicle.” A warrant for his arrest in Brazil was issued in June 2018, and he was considered “a fugitive from justice.”

The acting warden of Chester County Prison, Howard Holland, said at a news conference on Thursday that the circumstances of the escape were under investigation, declining to elaborate further.

Local and federal authorities, including the U.S. Marshals Service, have announced a $10,000 reward for information that leads to Mr. Cavalcante’s capture.

Ana Ionova contributed reporting from Brazil.