Amnesty International has reiterated its call for the Mexican federal authorities to thoroughly investigate the attack on human rights observers in the southern state of Oaxaca, which left two people dead and several injured.

Around 30 people in an International Caravan of Human Rights Observers were ambushed by armed men in the Indigenous Triqui area on Tuesday 27 April.

Jyri Antero Jaakola an observer from Finland, and Indigenous human rights defender Alberta Cariño -also known as Bety or Beatriz- (head of the organization CACTUS in Oaxaca state) were killed in the attack.

Journalist David Cilia and activist Noé Bautista, who were initially reported missing along with four others, were among those injured.

The group, which also included observers from Germany, Italy and Belgium, was on its way to document long-running human rights abuses in the Indigenous Triqui area, and to provide humanitarian assistance.

Journalists David Cilia and Erika Ramírez, and activists Fernando Santiago (of Mexican organization Brigadas Indigenas), David Venegas and Noé Bautista (of Oaxacan group VOCAL) hid for two days following the attack.

After waiting to be rescued, David Venegas and Noé Bautista walked for several hours to the town of Juxtlahuaca to seek help.

Some of those who survived the attack were told by the armed men to pass on the following message to Omar Esparza, husband of Alberta Cariño and leading member of the Oaxacan organizations CACTUS and MAIZ: "Tell him that he escaped this one, but he will be next."

Human rights groups in Mexico believe that the attack is related to previous threats from armed groups in the area.

The authorities did not go to the scene of the ambush until the day after, claiming that this was because of security concerns.

On 3 May, the victims of the attack filed a complaint at the National Human Rights Commission (Comisión Nacional de Derechos Humanos – CNDH) about the authorities' lack of action in the case and calling for a review of the investigation.

An Amnesty International delegation met with Federal officials on 29 April and asked them to intervene in the investigation.

On Friday, 30 April, the Federal Attorney General's Office (Procuraduría General de la República - PGR) took control of the investigation of the case

The Triqui region is mainly populated by Indigenous people and is one of the poorest and most troubled in the country.

For more than 30 years it has suffered inter-community conflicts in which scores of people have been killed.

State and federal authorities have rarely taken action to hold those responsible to account.

In April 2008, Felícitas Martínez and Teresa Bautista, two young Indigenous human rights defenders and journalists were killed. No one has been brought to justice for this crime. Both were colleagues of Alberta Cariño.