The FBI insists it is fully engaged in combating the threat of violence from white supremacists, but some former federal officials charge that the government is still coming up short in the face of a strain of American terrorism that now seems resurgent.

The weekend massacre at a Walmart and shopping center in El Paso has focused public debate once again on the issue, after federal prosecutors called it an act of domestic terrorism.

In recent congressional testimony, senior FBI officials said they were conducting about 850 domestic terrorism investigations — a decrease from a year earlier, when there were roughly 1,000. The category covers more than just racist violence, but officials say such motivations are a large part of their domestic terrorism caseload.

FBI’s Wray describes 100 domestic terrorism arrests

Yet by other measures, the threat of white nationalist violence appears to be rising. Between October and June, there were about 100 arrests of domestic terrorism suspects — and if that trend continues, the total for 2019 would outpace the prior year, when there were about 120 such cases. The year before that, about 150 domestic terrorism suspects were arrested.

“We will bring the full resources of the FBI to bear in the pursuit of justice for the victims of these crimes,” FBI Director Christopher A. Wray said in a statement issued late Sunday night.

In congressional testimony two weeks ago, Wray said the FBI is focused on the issue.

“We, the FBI, don’t investigate ideology, no matter how repugnant,” he said. “When it turns to violence, we’re all over it.”

That poses particular challenges in the world of white supremacists, who often do not belong to specific groups but instead find congregations of similar-minded people online and feed each other’s anger and hate.

That general lack of centralized groups or leaders among many white supremacists presents legal challenges to investigators, according to law enforcement officials. And in some cases, it shortens the time span from when a person may adopt extremist views and when they commit violence — what terrorism investigators call the “flash to bang.”

“The current racially motivated violent extremist threat is decentralized and primarily characterized by lone actors,” Michael McGarrity, an FBI assistant director, told lawmakers at a hearing in June.

So far, that appears to be the case for the suspect in El Paso, who investigators believe wrote a screed that professed admiration for white supremacist killers. Detectives have not yet found evidence indicating he was part of a larger group of conspirators, though they are still investigating.

Some veteran counterterrorism experts said the FBI and the federal government have done too little, despite concerns that have been building for more than a decade.

Dave Gomez, a former FBI supervisor who oversaw terrorism cases, said he thinks FBI officials are wary of pursuing white nationalists aggressively because of the fierce political debates surrounding the issue.

“I believe Christopher A. Wray is an honorable man, but I think in many ways the FBI is hamstrung in trying to investigate the white supremacist movement like the old FBI would,” Gomez said. “There’s some reluctance among agents to bring forth an investigation that targets what the president perceives as his base. It’s a no-win situation for the FBI agent or supervisor.”

Gomez said that reluctance stems in large part from the public criticism President Trump has launched against the FBI over the course of the bureau’s investigation into Russian election interference and the president’s ­conduct.

“I don’t think there’s any faith by the FBI right now that the Justice Department is an independent law enforcement organization,” he said. “I think the FBI is up to the challenge of investigating white nationalism and white supremacy as a domestic terrorism threat, they just have to be allowed to do it.”

An FBI spokesperson disputed those claims, noting that the agency assigns resources to reflect its assessment that domestic terrorists are a persistent threat. The official also denied that there is distrust between the bureau and the Justice Department, saying the two agencies work closely together.

The FBI said it “remains concerned that U.S.-based domestic violent extremists could become inspired by these and previous high-profile attacks to engage in similar acts of violence,” and asked the public to report any suspicious behavior they see online or elsewhere.

Daryl Johnson, a former analyst for the Department of Homeland Security whose 2009 memo warning of a growing threat of domestic terrorism led to a political backlash, said elected officials have been afraid to address the issue.

“Here we are 10 years later, this threat is still alive and well and in some respects is actually growing. It’s very concerning,” he said. “I’m still in disbelief that the federal government response seems to be lacking. People that we elect seem to lack political willpower to tackle this issue and call it out and do something about it.”

FBI data shows that hate crimes rose 17 percent in 2017, for their third consecutive increase, though officials cautioned that part of the increase is due to more police departments providing data than in previous years. FBI data on 2018 hate crimes is expected to be released in November.

Johnson, who wrote a book called “Hateland” about American extremists, said the government has pulled back grant programs for combating such violence at a time when they should have been beefing up such efforts.

“We’re in this heightened state of activity where we have mass shootings and bomb plots, and yet there’s no political willpower and everybody seems to be burying their head in the sand rather than try to tackle the issue,” Johnson said.

Experts say the way federal authorities investigate and prosecute terrorism suspects — particularly domestic terrorism suspects — hides the extent of the threat.

Under federal law, suspects plotting support for a group such as the Islamic State can be charged with the crime of providing criminal support to an overseas terrorist organization. There is no corresponding charge for providing material support to a domestic terrorism group.

Instead, domestic terrorist suspects who are not accused of a specific act of violence are typically charged with other crimes, such as drug or gun violations, and many of those cases are brought in state, not federal, court, meaning the general public often never hears that a domestic terrorism suspect has been ­arrested.

Some law enforcement officials have argued that Congress should enact a law that would make material support for domestic terrorism a crime. Others say that such a move could be struck down by the courts and that current laws are enough to deliver the most severe punishments possible.

In announcing Sunday that federal officials were considering filing hate-crime charges against the accused El Paso gunman, U.S. Attorney John F. Bash noted that such charges could carry a death sentence.