Despite President Donald Trump's promises to deport up to 3 million criminal immigrants, his administration has removed fewer people this fiscal year than during the same period last year, about 211,000 through early September, federal statistics show.

That's by far the lowest number of people deported in at least a decade.

The 12 percent drop has occurred even as the number of migrants Immigration and Customs Enforcement agents have arrested since Trump took office in January jumped by 43 percent to some 97,500 compared to the same time frame in 2016.

The end of fiscal-year statistics speak to the challenges the administration faces in removing large amounts of migrants from the interior of the country, where federal agents have far more constraints on how they can deport immigrants in the country illegally than at the borders, increasing the strain on an already-overwhelmed immigration court system.

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With a record backlog of more than 600,000 cases, it can take at least two years before a migrant appears in court and is ordered deported. Texas has the second-highest backlog in the nation, and cases have an average wait time of 829 days.

By contrast, immigration agents can, and do, deport most migrants apprehended within two weeks of arrival and 100 miles of the border through an administrative process known as expedited removal, which sidesteps the courts.

They can also simply return Mexicans and Canadians across the border, known as voluntary departure, without going through the time-consuming formal deportation process.

About two-thirds of the agency's removals involved immigrants arrested as they were trying to come in.

But the number of people trying to cross the U.S. border with Mexico plummeted significantly after Trump took office, falling by nearly half to almost 23,600 in February.

As the administration threatened to separate women and children at the border, and word of harsh enforcement spread, those numbers dropped further still to 15,800 in April, the lowest monthly figure recorded in at least 17 years.

Apprehensions have since steadily risen again to nearly 31,000 in August, according to Customs and Border Protection statistics.

But the number is still among the lowest in the past five years.

In fact, despite the blustering rhetoric about increasing enforcement at the border, sneaking across it is harder than ever before, according to a Department of Homeland Security report released this month. It estimated that as many as 85 percent of illegal border crossings are unsuccessful, up from 70 percent a decade ago.

Sarah Rodriguez, a spokeswoman for ICE, acknowledged that the record decline in apprehensions at the border had an impact on the overall number of deportations. But she noted that removals from elsewhere in the country increased by a third to nearly 52,200 between January and September when compared to the same period last year.

Of those who were arrested, but not yet deported, more than 70 percent are convicted criminals, she said, reflecting the "continued prioritization of enforcement resources on aliens who pose a threat to national security, public safety, and border security."

The agency's own statistics, however, show that by far the greatest increase came in the number of arrests of immigrants without criminal records, which almost tripled to about 28,000 since January when compared to the same period in 2016.

Rodriguez said that about 60 percent of those faced, but have not yet been convicted of criminal charges, though she didn't specify what kind.

The greatest number of non-criminal immigrants in the country were arrested in Atlanta, more than 2,500 between January and June, according to the most recent figures released. In that same time frame, the most with criminal records, about 7,000, were arrested in Dallas, followed by 5,900 in Houston.

The rise in arrests for immigrants without criminal records comes as agents have praised Trump for unshackling them and allowing them to deport anyone they find here illegally, rather than focusing on recently-arrived migrants and those with serious criminal histories as President Barack Obama ordered them to in 2014.

Obama did that after being labeled deporter-in-chief for removing a record 410,000 immigrants in 2012, but as hopes of comprehensive immigration reform fell apart in Congress he sought to protect immigrants who have been here a long time and prioritize criminals.

Under the Trump administration, every immigrant is again a focus for removal.

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John Sandweg, ICE's former acting director under the Obama administration, said the record deportation low this year, as agents have been ordered to deport everyone, shows that the strategy is a failure.

Even under Obama's targeted enforcement approach, Sandweg said the agency was able to easily apprehend about half a million immigrants who fit into its priorities from the interior of the country each year. But the system only had the capacity to deport around 150,000 of them.

"ICE has always had the ability to arrest far more people than they could possibly remove," Sandweg said. "What we really need is more immigration courts and immigration judges."

The drop in deportations also underlines the need for comprehensive immigration reform for some of the 11 million immigrants here illegally, he said.

"When we have a guy who is the most committed to having unfettered enforcement and he can't make a dent in the population, and in fact comes in under what we were doing with a far more targeted approach, that demonstrates the futility of trying to enforce your way out of the problem," Sandweg said.

The Trump administration faces far more obstacles in deporting large swaths of migrants than the Obama administration, said Randy Capps, director of research for U.S. Programs at the Migration Policy Institute, a think tank in Washington D.C.

Around 70 percent of immigrants detained by ICE are found after they are booked into local jails and their finger prints match with a Homeland Security database. ICE then asks the agencies to hold the migrants once they are eligible for release so it can pick them up.

But in the last few years several federal courts, though none in Texas, have ruled doing so violates the Fourth Amendment against unreasonable seizure.

Populous states, including California and Illinois, and about 260 counties and 30 cities stopped complying with the government's detainer requests, saying it is not within their authority and opens them up to lawsuits.

"This is a major barrier to increasing the scope of the enforcement system," Capps said.

The administration has tried to push back on so-called sanctuary cities, threatening to cut their federal funding. This week ICE arrested almost 500 immigrants from cities across the country that don't honor ICE requests, though none in Texas.

Gov. Greg Abbott has also pushed legislators to approve Senate Bill 4 to force local jurisdictions to comply with the detainers — though all, except Travis County in Austin, already do. A federal judge halted the law's implementation, but an appellate panel this week agreed that it can go into effect while the case is heard. As a result, Travis County announced it would honor the detainers once more.

Part of the strategy is signalling to migrants that they will suffer consequences for living here illegally, said Ira Mehlman, a spokesman for the Federation for American Immigration Reform, a Washington group advocating for reducing immigration.

"No one has seriously suggested that we can deport every person in the country illegally," Mehlman said. "The key is to take other steps to convince people, a) not to come, and b) if you are here, there's no reason to stick around."