William “Brock” Long, the top official at the Federal Emergency Management Agency, is facing a potential criminal probe related to his use of government vehicles, after an internal investigation into his travel was referred to U.S. attorneys for prosecution, according to administration officials and others familiar with the matter.

The development intensifies pressure on Long to step down and comes as he leads FEMA’s response to Hurricane Florence.

Long has been under scrutiny by the Department of Homeland Security’s inspector general for using the vehicles to travel between Washington and his home in Hickory, N.C., where his wife and children live.

Long’s predicament has put the White House in an awkward position, according to one administration official speaking on the condition of anonymity to discuss a possible criminal matter. President Trump has been pleased with Long’s performance at FEMA, and officials are worried about the potential fallout of removing him while large parts of North and South Carolina remain underwater from Florence.

Long arrived in North Carolina on Monday night and intends to survey damaged areas with Gov. Roy Cooper (D) on Tuesday. The storm has left at least 32 dead.

One person familiar with the probe said the Justice Department must determine whether Long’s trips warrant criminal charges. He has said that he did nothing unethical and that he remains committed to managing the hurricane response.

“I am not focused on this investigation,” he said in a statement Monday. “I am fully focused on those impacted by Hurricane Florence.”

Long’s referral to federal prosecutors was first reported by the Wall Street Journal.

DHS officials directed inquiries to the inspector general’s office, which did not respond, and to the Justice Department, which declined to comment.

The chairman of the House Oversight and Government Reform Committee, Rep. Trey Gowdy (R-S.C.), sent Long a letter Monday seeking documents related to his use of federal vehicles and communications with employees who may have joined him on personal trips to North Carolina.

Current and former FEMA staffers say a crucial aspect of the investigation will hinge on how prosecutors and administration officials interpret statutes that require Long to remain in communication with his agency and senior officials in Washington.

As the federal government’s primary emergency manager, the FEMA administrator is required by executive branch directive to have access to classified communication capabilities that help him support the continuity of government in the event of catastrophic events, including terrorist attacks and natural disasters.

Unlike Cabinet secretaries, however, Long does not have a 24-hour security detail.

“Let’s put some context on what the vehicles are that they’re talking about,” Long said Sunday during an interview with CBS. “So this job is incredibly complex. [On] my shoulders is Presidential Preparedness Directive 40, which means, you know, I have to make sure, FEMA has to make sure, that the executive branch of government works on its worst day at any given time regardless of what we see, and a lot of that is continuity of government. Those vehicles are to supply me with secure comms.”

But DHS officials and investigators appear to view his use of the vehicles and staff that accompanied him on his trips home as an unethical use of government resources. Long frequently left Washington to spend three-day weekends in North Carolina, driving hundreds of miles with aides who stayed in hotels at taxpayer expense, the Wall Street Journal reported, citing people familiar with the investigation. Officials also are investigating an accident involving a vehicle driven by one of FEMA’s staffers, according to the Journal.

Long’s supporters have characterized the investigation as a discrepancy between DHS and FEMA over the use of these specialized vehicles to meet those requirements.

“The administrator is on the clock 24 hours, seven days a week,” one former top FEMA official said. “There’s no vacations, no time off. So anywhere you go, you are responsible for coordinating the communications requirement.”

The critical question, according to another former FEMA official, is which part of Long’s trips home is viewed as potentially criminal.

“Is it the frequency of the trips? The SUV? The aides? What is the criminal act here? For the life of me, I can’t figure that part out. It seems more like a disciplinary issue than something that warrants a criminal investigation.”

Josh Dawsey contributed to this report.