RIO DE JANEIRO — Thousands of Brazilians took to the streets of Rio de Janeiro Saturday to demand answers in the death of city councilwoman and human rights activist Marielle Franco, whose slaying one month ago is seen by her backers as a political assassination.

Franco had been sharply critical of a security force takeover of policing in Rio. She and her driver Anderson Pedro Gomes were shot dead by assailants on March 14, while they were returning from an event on empowering young black women.

Authorities say surveillance footage shows two vehicles following Franco's car and they are trying to trace fingerprints from shell casings found at the scene and track cellphone signals. Police say at least 13 shots were fired at the councilwoman's car, four of them hitting her in the head. No arrests have been made.

Early Saturday, demonstrators stretched banners across some of Rio's iconic landmarks and spray-painted walls with slogans such as "Fight like a Marielle."

"Our desire to continue her legacy, to fight and demand justice only grows," said her sister Anielle Franco, who joined a large protest march later Saturday from Rio's downtown to the Estacio neighborhood, where the two were killed.

The sister said protesters don't just want to know "who killed her, but also who the person was who ordered this crime. We want to know who the person behind it is."

Several other cities across Brazil and around the world also planned events to commemorate the one-month anniversary of the death of Franco and Gomes, and to demand that their murders be solved.

Marcelo Freixo, a state representative in Rio and friend of Franco's, called her slaying a "crime against democracy."

"It's an attempt to quiet a young black woman from the favelas who was in politics," Freixo said.

Several international rights groups have called on Brazilian authorities to swiftly bring the assailants to justice.

"Society needs to know who killed Marielle and why. Every day that this case remains unsolved the level of risk and uncertainty surrounding human rights defenders grows worse," said Jurema Werneck, executive director at Amnesty International Brazil.

Local and international organizations have come together to start a fund in honor of Franco that will support black women who aspire to political leadership in Brazil. The Ford Foundation, the Open Society Foundations, the Ibirapitanga Institute and the W.K. Kellogg Foundation are among the donors.

Days before her assassination, Franco, an expert on police violence, accused officers of being overly aggressive in searching residents of favela slums. Her last post on Twitter called attention to the murder of a young favela resident.

"Another homicide of a young man that could be attributed to the police. Matheus Melo was leaving church when he was killed. How many others will have to die for this war to end?" she wrote.

Franco, who grew up in a favela in the Complexo da Mare, was known for her social work and was elected a council member in 2016.