MEXICO CITY — Mexican journalist Miroslava Breach spent her final days documenting murders.

She reported on the six people killed in a single night in her home state of Chihuahua, and on the assassination of a well-known environmental activist there. She wrote about the discovery of clandestine graves, about several police officers killed in an ambush, and about the bodies of three brothers that turned up, headless, in a rural mountain town.

On Thursday, Breach became a victim of the growing violence in Mexico that she chronicled so thoroughly. The 54-year-old mother of three was killed as she left her home in the capital city of Chihuahua — the third journalist slain in Mexico this month.

Breach was shot several times as she pulled her car out of her garage in the early morning, according to authorities. One of her children who was with her at the time was unhurt. Breach was rushed to a hospital, where she was pronounced dead.

A sign left at the crime scene said “tattletale,” according to La Jornada, the large national newspaper where Breach worked as a correspondent for 15 years. She also worked for several local newspapers and was reportedly starting her own news organization.

Authorities have not arrested any suspects in the case but said Thursday that Breach was likely killed because of her journalism, which cast a critical light not only on Mexico’s criminal groups but also the failings of its government.

“Miroslava denounced organized crime and also acts of corruption in the state Chihuahua, and that work is now our main line of investigation,” said Chihuahua’s governor, Javier Corral, who first met Breach in the 1990s.

In a televised news conference, he called her “courageous,” praised her “acute criticism” of society and the political class, and said the state would honor her with three days of mourning.

On Thursday local journalists protested at Chihuahua’s state congress, raising homemade signs that said: “Enough already.” According to local media reports, the microphone inside the chamber where the lawmakers meet was turned over to journalists, who demanded lawmakers do more to protect them.

According to a report released last year by the International Federation of Journalists, Mexico is the third deadliest country for journalists in the world, with 120 murders in the last 25 years. Only Iraq and the Philippines saw more journalists killed during that time.

In Mexico, this month has been particularly deadly.

On March 19, columnist Ricardo Monlui was shot twice as he left a restaurant with his wife and son in the Gulf Coast state of Veracruz. And on March 2, Cecilio Pineda Birto, a freelancer and the founder of La Voz de Tierra Caliente, was shot and killed at a car wash in Guerrero state.

Their deaths are a part an alarming rise in homicides across Mexico. There were 4,254 murder victims in Mexico in the first two months of this year, including 327 deaths in Chihuahua, more than at the beginning of any year since the government began releasing murder statistics.

More than a decade after Mexico launched a crusade against drug cartels — sending soldiers into local communities to battle gangs — a drug war still rages, and the murder rate is once again rising rapidly.

In a statement about Breach’s killing, the Committee to Protect Journalists said, “This wave of violence threatens citizens’ right to access vital information, and harms Mexico’s democracy by limiting public debate.”

The U.S. ambassador in Mexico, Roberta Jacobson, also expressed her condolences, via Twitter.

“May this and other crimes against journalists not remain in impunity to the detriment of freedom of the press,” she said.