Attorney General Jeff Sessions directed federal prosecutors across the country Tuesday to make immigration cases a higher priority and look for opportunities to bring serious felony charges against those who cross the border illegally — the latest in a string of controversial maneuvers to crack down on illegal entry into the United States and expand the Justice Department’s role in immigration enforcement.

In a three-page memo, Sessions directed each U.S. attorney to appoint a border security coordinator to oversee immigration prosecutions and to make immigration offenses — such as crossing the border illegally or harboring those who do — “higher priorities.”

He told prosecutors to consider whether they could bring felony charges against those who had entered the United States illegally multiple times and to evaluate whether they could charge illegal immigrants with aggravated identity theft, which carries a mandatory two-year prison sentence. He said law enforcement would no longer catch and release undocumented immigrants taken into custody at the border.

The directive signals a more aggressive posture on immigration issues than the Obama administration had taken. Advocates and legal analysts said it raises troubling questions about the Justice Department’s intentions and its use of resources.

Sessions, right, tours the U.S.-Mexico border with border officials in Nogales, Ariz. (Ross D. Franklin/AP)

“Which prosecutors and agents does he want to divert from the growing threats like terrorism, cybercrime, the opioid and heroin trade, organized crime and cartel activity?” asked Jenny Durkan, who served as U.S. attorney for the Western District of Washington from 2009 to 2014. “The ‘surge’ philosophy always requires taking agents, money and prosecutors from other priorities. In fact, the cost of satisfying Washington will reduce the ability of every U.S. attorney to address the greatest threats in their communities.”

Sessions touted the changes in Nogales, Ariz., on Tuesday as he toured the U.S.-Mexico border crossing there, about 70 miles south of Tucson. In a speech, he derided violent cartel and gang members and those smuggling people across the border, and he promised to take a stand against them.

He said illegal border crossings have declined significantly since President Trump took office — dropping 40 percent from January to February and 72 percent from December to March. Such crossings, though, also have decreased in recent years, and some researchers say demographic and economic factors have more influence than border security on the volume.

“As you know too well, this is a booming business. No more,” Sessions said. “We are going to shut down and jail those who have been profiting off this lawlessness — smuggling gang members across the border, helping convicted criminals re-enter the country, and preying on those who don’t know how dangerous and costly this journey can be.”

The Trump administration — and Sessions in particular — has taken a hard-line stance on immigration, alarming activists who say U.S. officials are testing legal boundaries and implementing policies contrary to American values.

Sessions last month threatened to strip federal funding from cities that did not communicate with Immigration and Customs Enforcement, and he signed a letter with Homeland Security Secretary John F. Kelly backing the practice of arresting undocumented immigrants at courthouses.

He also announced that he would expand a program to deport undocumented immigrants convicted of crimes after they serve their prison sentences, and he said Tuesday that he would increase the number of immigration judges to speed the removal of others.

Already, Sessions said, the Justice Department has posted 25 immigration judges in detention centers along the border, and he plans to add 125 more over the next two years. Those judges, though, handle deportation proceedings — not criminal cases.

“This is a new era,” Sessions said. “This is the Trump era.”

Paul K. Charlton, the U.S. attorney for Arizona from 2001 to 2007, said Sessions’s new directive would simply overburden the U.S. district court system, which is already struggling to handle the volume of immigration cases. He said that when he was U.S. attorney, his office had the highest number of prosecutions in the country, “yet the number of people entering illegally did not dramatically decrease.”

No one understands better than I do that prosecutions have a deterrent effect, but it’s not a solution. Prosecution and incarceration do not adequately address the real need, which is a reform of the immigration laws,” Charlton said.

As a senator, Sessions vigorously opposed immigration law reform.

Sessions’s memo offers blanket guidance to beef up enforcement, though it is somewhat short on specifics — omitting, for example, how to handle those coming to the United States with their children or those fleeing persecution, said Leon Fresco, who worked in the office of immigration litigation in President Barack Obama’s Justice Department.

The Justice Department, Fresco said, could slap asylum seekers with identity theft charges merely for using fake passports to get out of a country that otherwise would not have let them leave, or could jail parents who cross the border with their kids — breaking up the family. That would run counter to the assertion by the homeland security secretary that officials will not separate children from their parents.

“There’s nothing in the memo that talks about the need to be careful about that,” Fresco said.

Justice Department lawyers already spend a significant amount of time prosecuting immigration offenses. In fiscal year 2015, for example, such cases accounted for 29.3 percent of the entire federal caseload — behind only drug prosecutions, which accounted for 31.8 percent, according to data from United States Sentencing Commission. But the prosecutions had steadily declined in recent years and had fallen by 30.1 percent since 2011.

Gregory Z. Chen, director of advocacy for the American Immigration Lawyers Association, said the new Sessions memo seems to be an attempt at “scaring the public by linking immigrants to criminals despite studies showing that immigrants commit crimes at lower rates than the native born.”

“Sessions’s massive, wasteful effort to prosecute illegal border crossers will hurt families in America,” Chen said.