Two years ago, the Obama administration referred to the surge of Central American children and families coming into the U.S. as a "humanitarian crisis."

This year, it's worse – as Border Patrol agents apprehend even more Honduran, Guatemalan and Salvadoran immigrants claiming asylum. But due to a backlog in the courts, there is even less of a chance they’ll be deported.

"Where the backflow and choke point is occurring is in the immigration judge docket system -- 800,000 on the dockets right now," said former Deputy Commissioner of U.S. Customs and Border Protection David Aguilar. "So, that backlog, that inability to basically send these people back, remove them back to their countries of origin, is causing a draw of more of these people coming into the country."

According to the latest CBP figures, agents detained 27,754 unaccompanied minors from Central America in the first six months of the fiscal year, almost double last year's total of 15,616 and just shy of the 2014 record of 28,579.

The numbers for immigrants traveling as families is even higher, with 32,117 apprehended -- almost triple last year's total of 13,913 and well above the 2014 “surge” figure of 19,830.

Taken together, the exodus from Central America represents the largest mass immigration to the U.S. since the Mariel boatlift out of Cuba in 1980.

"These people are allowed to remain here. They are given employment authorization -- therefore, they are sending a message back home 'come on over'," Aguilar said.

There are push-and-pull factors to any mass immigration, but agents and immigration experts point to several drivers.

Because of a lack of jail space and the backlog in the courts, it can take five years or more before an illegal immigrant who claims asylum actually sees a judge to have their case resolved. In the meantime, the women, children and families are usually released to a relative or aid group where they integrate into the country; few are ever deported. This narrative creates an incentive for more to come, despite administration efforts to dissuade Central Americans from crossing in.

Violence, poverty and lawlessness in El Salvador, Honduras, and Guatemala is forcing families looking for a better life to leave. Gangs and drug cartels often target children, turning to the boys to traffic narcotics and the girls for prostitution.

Agents say many detained immigrants claim they are trying to 'beat the election.' In other words, they want to get in as soon as possible -- in case Donald Trump is elected and builds a wall along the Mexican border, or Hillary Clinton is elected and pursues comprehensive immigration reform. They would want to be included under any such reform push.

Agents are concerned the system, though, is so overwhelmed that few immigrants’ criminal records are adequately checked, endangering the public.

"We don't know who we're releasing and we don't know what they're capable of," said Shawn Moran, a Border Patrol agent in California and a member of the union. Fellow agent and union representative Art Del Cueto echoed, "You can't find out if they've murdered, you can't find out if they've molested minors."

The immigration rush isn't just at the southern border. According to the Department of Homeland Security, almost 500,000 immigrants who entered the U.S. legally last year overstayed their visa. Yet, according to Immigration and Customs Enforcement, the agency removed fewer than 3,000 overstays, or less than 1 percent.

"Over 40 percent of the illegal entries, illegal people that stay in the United States come in by visa and just plain overstay," said Aguilar. "So the ability to locate them, to identify them and put them in removal proceedings and very critically remove them as quickly as possible from the country is critical."

The U.S. does not have the means to easily track visa overstays, despite a law and millions of dollars appropriated by Congress to do so.