Even as evidence mounts that illegal immigration flows have fallen to their lowest level in years, the union that represents border patrol officers is saying the government could be doing much better at stopping people who still try to cross the U.S. southwest border.

In a series of recent interviews, union officials described a difficult, often perilous job in which they are struggling at times to keep up with migrants seeking to outwit the government’s heightened security measures. They said morale among agents has plunged, partly because of the executive actions shielding millions of illegal immigrants from deportation that President Obama announced last year.

“We know there is a lot of traffic still getting through the border,” said Shawn Moran, vice president of the National Border Patrol Council, which represents more than 16,000 agents. Moran, a 17-year agent based in San Diego, and other union officials criticized U.S. Customs and Border Protection — which includes the border patrol — as inefficient and top-heavy with supervisors.

R. Gil Kerlikowske, commissioner of U.S. Customs and Border Protection, praised the dedication of border patrol agents and said “in many ways, I get their issues and frustrations.” But after the Department of Homeland Security’s more than decade-long crackdown on southwest border security, he said, the agents have plenty of staff and technology to do their jobs.

“If some of these folks are so unhappy, they really need to reassess what they do and where they are,” Kerlikowske said. Customs and border protection is part of DHS.

The debate comes as a number of indicators show that immigration flows are falling, especially from Mexico. Researchers say far fewer Mexicans are planning to cross the border than in years past, and the overall U.S. illegal immigrant population — which more than tripled, to 12.2 million, between 1990 and 2007 — has dropped by about 1 million, according to demographers at the Pew Research Center.

Homeland security officials point to measures they have taken since the George W. Bush administration, including more than doubling the border patrol’s size and spending billions on new technology, as driving the trends. “Put simply, it’s now much harder to cross our border and evade capture than it used to be — and people know that,” said DHS secretary Jeh Johnson said in an October speech. He has repeatedly spoken publicly about the importance of border security.

Some experts agree, while others instead point to changes in Latin America, such as the improving Mexican economy. Border security is critical to the debate over immigration reform in Washington, with congressional Republicans saying the southwest frontier must be more secure before they will consider legalizing illegal immigrants already in the United States.

Among the key indicators cited by DHS is the rapid decline in apprehensions at the border. Since 2000, when more than 1.6 million border crossers were stopped, those numbers have plunged to around 400,000 per year, and they are down 28 percent in the first part of fiscal 2015 compared with last year.

But union officials say those figures don’t mean much because they don’t chart people who successfully make it into the United States.”This notion that DHS is saying the border is more secure than ever — they don’t have any evidence of that,” said Brandon Judd, the union’s president and a 17-year agent based in Maine.“It’s just smoke and mirrors.”

Chris Cabrera, a 13-year agent based in Texas and a union official, said Obama’s executive actions have sent mixed messages to the agents in the field. Those actions have faced resistance in the courts, including the decision Tuesday by a federal appeals court to keep one of the president’s signature immigration efforts from moving ahead.

“Border crossings are usually tied with perceived amnesty,” Cabrera said. “If people believe they will get some type of relief or a free ride, the floodgates open.”

Kerlikowske suggested that his agents focus on law enforcement, rather than politics. “You don’t get to control certain things’’ he said. “I’m not the judge, jury and executioner and commissioner, and certainly at their level in the border patrol, they’re not either.”