(CNN) -- Gunmen raid trains, demanding money and taking prisoners. Government officials run human trafficking rings. Police find the bloodied bodies of slain immigrants on an abandoned ranch.

Allegations from authorities paint a bleak picture of life for thousands of Central Americans heading to the United States through Mexico.

The discovery and detention of more than 500 illegal immigrants crammed into two semi-trucks in southern Mexico this week was a startling reminder of the desperation that drives many -- and the dangers they face.

"They are very vulnerable. They face risks of all kinds," said Diana Martinez, a lawyer with Sin Fronteras (Without Borders), a Mexican immigrant rights organization.

Experts say many Central American migrants passing through Mexico on their way to the United States are victims of violence along the Mexico-U.S. border, but the journey is perilous from the moment it begins. And the danger is growing as Mexico's drug gangs expand their reach.

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"Now (immigration) is all about organized crime. This is really controlled either directly by the drug trafficking cartels or by immigrant smuggling rings big enough to negotiate with the cartels. ... We're seeing large-scale violence that we didn't see 20 years ago," said Andrew Selee, director of the Mexico Institute at the Woodrow Wilson Center for International Scholars in Washington.

Criminals regularly target thousands of migrants passing through Mexico, Amnesty International said in a statement Wednesday, noting that immigrants "face a variety of serious abuses from organized criminal gangs, including kidnappings, threats and assaults."

In August, authorities found the bodies of 72 slain immigrants from Central and South America on an abandoned ranch near the Mexico-U.S. border.

At least 11,333 migrants were kidnapped in the six-month period from April through September 2010, according to an investigation by Mexico's National Commission for Human Rights. A map of high-risk areas for immigrants in the commission's February report highlights areas across the country along the unofficial path known as "the migrant's route."

Mexico no longer has a nationwide passenger rail system, but thousands of immigrants hitch rides on freight trains heading north. They huddle on rooftops and cram into spare spaces between cars.

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As reports of drug-gang raids and kidnappings become increasingly common, many call it "the train of death." But they say the journey is worth the risk.

"As I tell my friends, we're all going to die one way or another," said Carlos, an immigrant from El Salvador who stopped at a shelter in central Mexico this week. "The thing is, we have to face reality. Because they tell you so many things about what's happening up there, but the need is very great, and with the help of God nothing is impossible."

Others said threats at home are far worse.

"In our countries, the situation is very hard. They're killing for 200 quetzales (about $25), and one can no longer have a business, have a bus, a truck, because if you don't pay the taxes, they attack, and this is the motivation of all the Central Americans who are trying to reach the United States," Guatemalan immigrant Eddie Marroqui said.

X-rays taken this week at a highway checkpoint in the southern Mexican state of Chiapas -- which borders Guatemala -- found more than 500 immigrants crammed into two semi-trucks. Most of them hailed from Central America, and they had paid $7,000 each for passage to the United States, authorities said.

"It points to how profitable these operations are for the organized syndicates that move people for profit. And it also points to the need for governments to really target that crime in their legislation," said Demetrios Papademetriou, president of the Migration Policy Institute in Washington.

Recently Mexican authorities have identified another target: immigration officials.

The commissioner of Mexico's National Migration Institute announced this month that 200 agents had been relieved of duty, and 40 of them were subject to criminal prosecution, after evaluations aimed at stamping out corruption and ties to organized crime.

In March prosecutors announced that 36 Mexican immigration agents in Cancun had been detained for alleged human trafficking.

"I think the Mexican government has finally become concerned about how violent immigrant smuggling has become. We've seen them taking the first step to try to clean up some of the endemic corruption in the immigration service," Selee said.

Amnesty International said Wednesday that many immigrants "face abuses at the hands of Mexican authorities, including excessive use of force and arbitrary detention."

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In a speech Thursday, Mexico's interior minister said the country must improve security for migrants and Mexicans alike, and strengthen state institutions in order to bring criminals to justice. He also vowed to step up infrastructure and security along Mexico's southern border with Guatemala.

His remarks came several weeks after Mexican lawmakers approved a new immigration law that officially decriminalized undocumented immigration in Mexico. Officials said the law is aimed at protecting migrants and cracking down on human trafficking.

But the law has not yet been published, and it is unclear how it will affect immigration enforcement.

"We are still chasing immigrants as though they are a threat. We are a racist country," said Perseo Quiroz of Sin Fronteras, the Mexican immigrant-rights organization.

But Mexican government officials have a different take. To describe immigration operations like this week's semi-truck discovery in Chiapas, they use words like "rescued," "secured" and "returned."

"Basically they were arrested and deported," Selee said. "It's a real contradiction. On the one hand they're trying to show that they have control of their borders. On the other hand, they don't want to become like the United States, a country which many Mexicans see as expelling foreigners."

CNN's Krupskaia Alis and CNNMexico.com contributed to this report.

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