The increase in the number of migrants crossing Mexican territory on their way to the United States has led to more threats against human rights activists, the Rev. Pedro Pantoja Arreola, director of the Casa del Migrante shelter in Saltillo, told Efe.

Activists have been harassed and received threats, but it is not known who is behind the harassment, Pantoja said.

A complaint was filed a few days ago with state prosecutors, the shelter director said.

"We were at the prosecutor's office to complain about the threats. What happens is that it's not just about the recent threats, but also that we are dealing with a situation of great violence," the priest said.

The flow of Central American migrants into Saltillo, the capital of the northern state of Coahuila, has surged in the past few months, "making it attractive for organized crime groups, which recruit them to commit crimes for them," Pantoja said.

"Since March, we have experienced a human avalanche of more than 350 people daily. What a bonanza for the people traffickers! And they were like vultures on a house," the shelter director said.

Criminal organizations have men posing as migrants going into the shelter to recruit and run extortion rackets against those inside, Pantoja said.

It is often useless to provide assistance to the Central Americans because the majority of them end up in the hands of Los Zetas, considered Mexico's most violent drug cartel, or corrupt officials, who work with criminal organizations, the human rights activist said.

Casa del Migrante was accused of helping migrants commit crimes in the wake of the Dec. 24 murder of a Honduran migrant by another migrant in Saltillo, Pantoja said.

The shelter director denied the allegations.

A supposed relative of the young Honduran murdered on Christmas Eve later phoned and made threats, the priest said.

"I think there are many things behind this because the criminalization of our work continues after this and the telephone call," Pantoja said.

Coahuila Attorney General Homero Ramos Gloria said an investigation was opened to try to determine who is behind the threats against Casa del Migrante.

An estimated 300,000 Central Americans undertake the hazardous journey across Mexico each year on their way to the United States.

The trek is a dangerous one, with criminals and corrupt Mexican officials preying on the migrants.

Gangs kidnap, exploit and murder migrants, who are often targeted in extortion schemes, Mexican officials say.

Central American migrants follow a long route that first takes them into Chiapas state, which is on the border with Guatemala, walking part of the way or riding aboard freight trains, buses and cargo trucks. EFE