(CNN) -- A federal judge in Mexico has sentenced four people to a minimum of 16 1/2 years each behind bars for human trafficking, the Mexican attorney general's office said Tuesday, marking a rare conviction in a country struggling to get a grip on the illegal trade.
The investigation began because of a tip from the U.S. Immigration and Customs Enforcement (ICE) agency, authorities said.
Agents rescued four women being forced to work as prostitutes in Miami, Florida, according to a statement from the Mexican attorney general's office. An ICE spokeswoman said U.S. agents identified two of the victims in Miami and that the other two were identified by authorities in Mexico.
Their children were being held in Tenancingo, a municipality in the southern Mexican state of Tlaxcala, and the women's contact with them was conditioned on their sending money to their handlers, the statement said.
On June 7, 2010, Mexican authorities raided the home in Tlaxcala, arresting the four suspects.
Fausto Velazquez Zompantzi, Jaime Velazquez Zompantzi and Severiana Zompantzi Rojas were sentenced to 16 1/2 years in prison each for the parts they played in the crime. Jorge Velazquez Zompantzi was sentenced to 18 1/2 years. In addition to trafficking, he was also found guilty of possessing a firearm licensed exclusively to the military.
The women's children were sent to be with their mothers in the United States, according to the attorney general's office.
The statement said the case represents the first time the attorney general's office of the Special Prosecutor for Organized Crime, also known as SIEDO, obtained a conviction and sentence for a human trafficking crime.
Mexico, like many countries, struggles to investigate, prosecute and punish trafficking offenders.
The U.S. State Department released its 2011 report on human trafficking Monday, assessing efforts by 184 governments worldwide to fight sexual exploitation, forced labor and moder-day slavery.
The annual report, considered one of the most comprehensive analyses of worldwide human trafficking, ranks countries in three "tiers." Mexico was put in the middle tier.
Notably, the reported cited the municipality of Tenancingo as a major source for Mexican sex trafficking victims exploited within Mexico and in the United States.
Women, children, indigenous people and undocumented migrants are especially vulnerable, the report read.
While noting some of the recent strides made by the Mexican government in countering trafficking, particularly at the federal level, the study also cited reports that some local law enforcement officials tolerate and are sometimes complicit in the illegal trade.
"Given the magnitude of Mexico's trafficking problem ... the number of human trafficking investigations, prosecutions, convictions, and sentences remained low, and government funding for victim services remained inadequate," it read.
The State Department says 27 million people are victims of human trafficking worldwide, and approximately 100,000 of those victims are in the United States.