Mexico City (CNN) -- The office of Mexico's top prosecutor is investigating the slaying of the sons of two well-known journalists
Brothers Alfredo David Paramo, 20, and Alejandro Paramo, 21, were found dead early Saturday morning in the capital of the border state of Chihuahua. Authorities found 9mm bullet shell casings at the scene, the state-run Notimex news agency reported.
Their parents -- David Paramo, a financial journalist, and Martha Gonzalez, editor-in-chief of the El Peso de Chihuahua newspaper -- are prominent journalists in Chihuahua, a Mexican state known as one of the country's most violent.
Local media reported that state prosecutors have said the couple's work had nothing to do with their sons' deaths.
But Articulo 19, a nonprofit group that pushes for press freedom in Mexico, says it's too soon to rule that out.
"They (prosecutors) did that without presenting proof and based on unofficial accounts given to the press," the organization said in a statement, calling for an impartial investigation of the case.
The case drew attention from many of the country's top political leaders and journalists over the weekend, with Mexican President Enrique Peña Nieto among those who expressed condolences in Twitter posts.
In another post, Peña Nieto said he had ordered Mexico's attorney general's office to help local authorities in their investigation.
A spokesman for state prosecutors said Sunday that authorities were pursuing a number of lines of investigation, focusing on the victims' social and family relationships.
The brothers returned home shortly after 4 a.m. on Saturday, then one of them told their mother they had to go out again, Carlos Gonzalez, the spokesman for state prosecutors, told Mexico's Milenio TV.
"We do not know for what reason, they left the house, and within minutes they were followed and became victims," he said.
At least 14 journalists in Mexico were killed in retaliation for their work in the country from 2006-2012, marking former President Felipe Calderon's tenure as "one of the deadliest on record for freedom of the press anywhere in the world," according to the New York-based Committee to Protect Journalists.
"Across Mexico, authorities at the state and local level have a dismal record of solving journalist murders," the organization said in a statement last month. "CPJ research shows that officials have been known to attack the reputation of the victims, either directly or through leaks to the press."
CNN's Catherine E. Shoichet, Rafael Romo, Krupskaia Alis and CNNMexico.com contributed to this report.