RIO DE JANEIRO — Lino Oviedo, a candidate in Paraguay’s presidential election and one of the country’s most polarizing political figures, was killed in a helicopter crash on Saturday night while returning from a rally in northern Paraguay, government officials said Sunday.

The death of Mr. Oviedo, 69, opens new uncertainty in Paraguay, where President Fernando Lugo was ousted last year. After the authorities confirmed Mr. Oviedo’s death and called it an accident, officials in Mr. Oviedo’s party, the National Union of Ethical Citizens, immediately questioned whether he had been assassinated.

Mr. Oviedo, a retired general who had led the Paraguayan Army, had a tumultuous political career. He initially gained prominence in 1989, when he helped topple Gen. Alfredo Stroessner, the dictator who ruled Paraguay for 35 years.

Mr. Oviedo fled the country in 1999 — seeking exile first in Argentina and then in Brazil — after being charged with organizing an aborted coup in 1996 against Juan Carlos Wasmosy, who was then Paraguay’s president.