BUENOS AIRES—The prosecutor who last week accused Argentine President Cristina Kirchner of working with Iran to subvert a probe into a 1994 terror bombing was found dead in his apartment on Sunday, less than a day before he was to testify in Congress over the unresolved crime.

Alberto Nisman’s body was discovered by his mother and a custodian inside a bathroom at his home on the 13th floor of an apartment tower here in this capital city with a gunshot to the temple, Viviana Fein, the prosecutor in charge of the case, said on Monday.

Officials found a 22-caliber handgun beside Mr. Nisman, 51 years old. An autopsy determined he fired the gun at himself at point-blank range, Ms. Fein said. He didn’t leave a suicide note and there were no signs of forced entry, she said.

“Suicide, in addition, in all cases leads, first, to shock and then questions,” Mrs. Kirchner said in a statement. “What was it that led a person to make a terrible decision and take their life?”

The timing of the death made many Argentines suspicious. On Wednesday, Mr. Nisman filed a criminal complaint that accused Mrs. Kirchner, Foreign Minister Héctor Timerman and others of conspiring to cover up a probe into Iran’s alleged involvement in the attack on a Jewish community center here that killed 85 people, the worst attack targeting Jews since World War II.