The FBI headquarters in Washington, Feb. 3, 2012. Credit: Manuel Balce Ceneta | AP

WASHINGTON — Lawmakers worried about domestic terrorism in the United States want the FBI and the Justice Department to provide a complete accounting of the threat, after recently disclosed data show arrests in such cases now outpace those in international terrorism probes.

Rep. Bennie Thompson, D-Mississippi, chairman of the House Homeland Security Committee, wrote to senior law enforcement officials this week seeking a briefing from law enforcement officials. The letter references internal FBI figures, which were first reported by The Washington Post last week, showing that during the 2017 and 2018 budget years, authorities arrested more domestic terrorism suspects than those inspired by Islamic extremists.

“Recent high-profile acts of violence and attempted violence have underscored the urgency of understanding and developing solutions to the evolving threat of domestic terrorism,” Thompson wrote to Attorney General William Barr and FBI Director Christopher Wray.

Thompson’s letter was distributed to the media Thursday, hours before 49 people were killed in a terrorist attack targeting mosques in New Zealand. Authorities there are examining a manifesto whose author, officials say, expressed contempt for Muslims and immigrants, and exulted notorious right-wing extremists, including Dylann Roof, who in 2015 killed nine black churchgoers in Charleston, South Carolina.

The congressman’s request comes amid a growing public debate over whether federal law enforcement is too focused on Islamic terrorism and not paying enough attention to the rise in far right-wing extremism.

Thompson’s letter is his third in recent months complaining to the FBI that he wants more information about the current state of the threat of domestic violent extremists.

The latest letter, dated Wednesday, requests a classified briefing to his committee later this month. Thompson also said he plans to hold a public hearing on the issue soon.

According to FBI figures shared with The Washington Post, in the 2017 budget year there were about 110 people arrested in the U.S. after being investigated for actions inspired by foreign terror groups such as the Islamic State and al-Qaida. Of those, about 30 faced terrorism charges. The rest faced gun, drug, fraud or immigration charges.

Last year, out of about 100 such arrests, only nine defendants faced terrorism charges — a drop-off owed in part to a decline in the number of people attempting to travel overseas to join the Islamic State, the senior law enforcement official said.

In the 2017 budget year, FBI investigations led to the arrest of about 150 domestic terrorism suspects, according to law enforcement officials. The following year, the figure was about 120.

But because federal law does not designate domestic groups as terrorist organizations, no corresponding terrorism crimes apply to such suspects. And while the number of such arrests fell last year, the senior law enforcement official emphasized that, overall, more domestic terrorism suspects are being arrested.

About one in four counterterrorism arrests are made by state and local authorities — not the FBI, the senior law enforcement official said.

The figures underscore how the majority of the FBI’s counterterrorism work is done under the radar, even in cases which end in arrests. Every year, thousands are investigated. Hundreds are charged with crimes. But the press and public see only dozens.