RIO DE JANEIRO — Gleydson Carvalho’s radio program was on the air in the provincial beach city of Camocim when two gunmen burst into his studio. During a musical interval, they subdued the receptionist and told a technician to stay quiet. Then they did their work, unloading three rounds and killing Mr. Carvalho, a journalist known for crusading against political corruption.

The shooting of Mr. Carvalho on Thursday sent shock waves through Ceará, a state in northeastern Brazil, while raising alarm among human rights groups as part of a spike in execution-style killings of journalists outside major urban centers around the country.

“It all happened very fast,” Ricardo Farias, a technician at the radio station who witnessed the killing, said in televised comments in Ceará. “He received threats that they were going to kill him, and he would say on the air that he was threatened but unafraid of anyone,” Mr. Farias added. “I always said he shouldn’t act like that.”

At least three other journalists have been killed in Brazil this year in retaliation for their work, bringing the number of such cases before the killing of Mr. Carvalho to 16 since 2011, according to the Committee to Protect Journalists, a news media advocacy group in New York. The circumstances surrounding six other cases remain unclear.