People take part under heavy rain in the "Marcha del silencio" (March of Silence) called by Argentine prosecutors in memory of their late colleague Alberto Nisman in Buenos Aires Wednesday. (Juan Mabromata/AFP/Getty Images)

A month after prosecutor Alberto Nisman was found dead in his bathroom, thousands of Argentines marched Wednesday evening to protest the unsolved mystery and criticize the government’s handling of the case.

Chanting “Justice!” and carrying black-lace Argentine flags and banners with messages such as “Truth cannot be killed,” the crowd held up umbrellas against torrential rain as they walked from Congress to the downtown Plaza de Mayo.

Despite weeks of twists and turns, authorities are no closer to determining whether Nisman took his own life last month or was murdered. But the frustration and outrage at President Cristina Fernández de Kirchner’s government has continued to build, with protests in the capital and in other cities.

“His death has moved us,” said Fedora Ines Mirochnik, a protester whose father was killed along with 84 others in the 1994 bombing of the Jewish community center in Buenos Aires. Nisman devoted his career to investigating that attack and came to the conclusion that Iranian officials had planned and financed the bombing. Last month, he filed criminal charges against President Fernández and her aides, alleging that they had conspired to cover up Iran’s involvement. But a day before he was to testify to Congress about those allegations, he was found dead in his apartment from a gunshot to the head.

“I have no doubt he was murdered,” Mirochnik said, echoing a common sentiment among Argentines. “He couldn’t even breathe without thinking about the case. He was waiting for the right moment to put forward his charges, but they silenced him.”

A woman holds a signs with prosecutor Alberto Nisman's portrait in Rosario, Argentina during a march simultaneous to the "Marcha del silencio" (March of Silence.) President Cristina Kirchner urged Argentines to be on guard Wednesday ahead of a mass protest over the mysterious death of Nisman who had accused her of a cover-up in his probe of a 1994 bombing. (Str/AFP/Getty Images)

Fernández and others have vehemently denied any secret deal with Iran or involvement in Nisman’s death. Foreign Minister Héctor Timerman on Tuesday read a letter he had sent to U.S. Secretary of State John F. Kerry saying that Argentina would not tolerate being a “theater for operations of politics, intelligence or, even worse, more serious actions,” by other countries he left unnamed. Fernández supporters have accused the opposition of using Nisman’s death to attack the government. With a presidential election coming up in October, rival politicians have seized on the case.

“I don’t care if the government says that we are here to destabilize them,” said María Gómez, a 55-year-old protester who attended the march. “We want an independent justice system, a fully functioning democracy.”

After Nisman’s death, a new prosecutor took over the complaint against Fernández and her allies and has said the case will move forward. During the search for evidence, a draft arrest warrant for Fernández was found in the trash outside Nisman’s apartment building in the Puerto Madero neighborhood, but it is unclear whether the new prosecutor will attempt to push for the president’s arrest.

Either way, the case has become a major scandal for Fernández’s government and dominated the Argentine media for the past month. Wednesday’s protest, organized by fellow prosecutors as a tribute to Nisman, was the largest demonstration so far. Nisman’s ex-wife, the federal judge Sandra Arroyo Salgado, attended along with the couple’s eldest daughter. To avoid violence, Argentine police were ordered not to carry weapons while guarding Wednesday’s march.

“I think that Nisman has become the excuse for people to get together and ask for democratic institutions — which we don’t have in Argentina,” Mario Cimadevilla, an opposition senator, said in an interview. “This march becomes a chance to reflect deeply on an important issue: Why does the justice system in Argentina never promote investigations into those who have a strong hold on power?”

During a speech Wednesday, Fernández did not refer directly to the protest but defended her government in vague terms.

“We’re not isolated,” she said. “We live in a world full of interests that want some to be subordinate and others to rule, and they are put face to face with a government like this, that does not allow anyone else to set its agenda.”

Over the past month, bizarre new details have added layers of confusion to the case. A 26-year-old waitress, Natalia Fernández, claimed in an interview with the Argentine newspaper Clarín that police ushered her and a friend into Nisman’s apartment the night of his death to be objective witnesses to the crime scene. She described chaos: dozens of people moving in and out of the apartment, authorities drinking yerba mate and eating mini-croissants, doormen making coffee in the kitchen. The lead investigator on the case, Viviana Fein, has denied Natalia Fernández’s claims.

On Sunday, a woman’s body was found badly burned along with a five-liter tank marked “ethyl alcohol” next to a power station not far from Nisman’s building in Puerto Madero. Authorities have not claimed that the unusual death is linked to Nisman’s case, but the Argentine media have seized on it as another mystery to unravel.

Arroyo, Nisman’s ex-wife, has been critical of the government’s handling of the case, saying that Fein has been sharing too many details with the media. Arroyo called for an independent international commission to oversee the investigation.

“I believe that Nisman was working in the service of the country, trying to clarify an important case, and this cost him his life,” said Elma Borda, a 70-year-old nun of the Carmelite order, who said she was attending her first protest in Argentina. “Today is a tribute not only to Nisman, but to all the people of Argentina.”

Partlow reported from Mexico City.