A group representing FBI agents warned Thursday that the partial government shutdown is threatening national security as thousands of federal law enforcement professionals, working without pay, grow anxious that personal financial hardships may jeopardize their security clearances and as furloughs of their support staffs slow investigations.

The shutdown is the result of President Trump’s insistence that more miles of border wall be built in the interest of national security — to keep migrants and drugs from entering illegally — and Democrats’ refusal to go along with his demands for $5.7 billion in wall-construction funds. With the shutdown well into its third week, groups representing government employees ranging from those who patrol borders and guard courthouses to those who make undercover drug buys have expressed alarm that the political drama has reduced them to bargaining chips while they continue doing dangerous jobs that keep Americans safe.

“It’s uncharted territory, as this shutdown is going to be the longest in history,” said Thomas O’Connor, president of the FBI Agents Association. “For special agents, financial security is national security.”

Of particular concern to FBI personnel is Friday’s missed paycheck — their first since the shutdown began in late December.

In a letter to the White House and lawmakers, FBIAA leaders wrote that their agents “are subject to high security standards that include rigorous and routine financial background checks. . . . Missing payments on debts could create delays in securing or renewing security clearances, and could even disqualify agents from continuing to serve in some cases.”

O’Connor said FBI investigations already are being affected. No one at the FBI is getting paid, but investigators are still working while much of their support staffs, including some surveillance experts, are not, O’Connor said. The FBI lab in Quantico, Va., has faced significant staff reductions, and the money available for investigative expenses such as undercover drug buys is dwindling, he said.

“Operations are being hindered,” O’Connor said. “This situation is not sustainable.”

The FBI has nearly 13,000 special agents, and already some families are making tough choices to manage household finances. Several current FBI employees said Thursday they worry that there is no sign of the shutdown being resolved soon and that, as a result, they could miss not one but two paychecks.

Dave Gomez, a retired FBI supervisor whose wife works at the bureau, said he is pulling money from his retirement savings to make sure they can pay their mortgage.

Gomez, who experienced a shutdown during the Obama administration, said that back then he told his agents not to worry, because shutdowns last only a week or two, not long enough to miss a paycheck.

“What’s happening now is a shock for a lot of agents, and I think you will see a lot of agents stressed about missing a paycheck,” he said. “FBI agents aren’t different than other Americans in that a lot of people live paycheck to paycheck.”

The shutdown affects all the federal law enforcement agencies within the Justice Department and the Department of Homeland Security. U.S. marshals guarding the trial of accused drug kingpin Joaquín “El Chapo” Guzmán in New York, amid extremely tight security, are working without pay. Agents with the Secret Service, the Drug Enforcement Administration and the Bureau of Alcohol, Tobacco, Firearms and Explosives, as well as Immigration and Customs Enforcement personnel and Customs and Border Protection officers all are working without pay and much of their support staffers.

“The thing that concerns us the most,” said Patrick O’Carroll, executive director of the Federal Law Enforcement Officers Association, “is with all the ‘nonessential’ personnel that are not showing up. With active investigations and arrests, when you start taking out the analysts, you’re losing a big part of that.”

FLEOA has warned its members “to be extra vigilant and cautious performing your duties” because of the increased risks associated with not having support staffers available.

The shutdown debate is more complicated within the Border Patrol, because many of those agents support Trump’s demand for more wall construction.

Border Patrol union leaders appeared at the White House last week to show their support for Trump, despite agents working without pay. While the president continues to have broad support within the Border Patrol, some agents say there is growing worry among the rank and file about missed car payments and late mortgages. Many of those who are assigned to remote border towns are their families’ sole breadwinner.

“People have started to contact creditors,” said one agent in Arizona who spoke on the condition of anonymity because he was not authorized to comment to reporters.

Morale is sinking because agents are already under strain after a busy period along the border, with record numbers of families coming across, according to one agent in South Texas who also spoke on the condition of anonymity. This agent, also not authorized to comment publicly, faulted Democrats for the shutdown.

The National Border Patrol Council’s fealty to the president is not shared by the National Treasury Employees Union, which represents the blue-uniformed Customs and Border Protection officers assigned to U.S. ports of entry, an even larger workforce.

“If employees are working, they must be paid — and if there is not money to pay them, then they should not be working,” NTEU President Tony Reardon said in a statement Wednesday after his union filed a suit alleging that federal laws that force workers to stay on the job without pay are unconstitutional.