WASHINGTON — Hackers have targeted the voter registration systems of more than 20 states in recent months, a Homeland Security Department official said Friday.

The disclosure comes amid heightened concerns that foreign hackers might undermine voter confidence in the integrity of U.S. elections. Federal officials and many cybersecurity experts have said it would be nearly impossible for hackers to alter an election’s outcome because election systems are very decentralized and generally not connected to the internet.

The official who described detecting the hacker activity was not authorized to speak publicly on the subject and spoke to the Associated Press on condition of anonymity. It was unclear, the official said, whether the hackers were foreign or domestic, or what their motives might be.

The FBI last month warned state officials of the need to improve their election security after hackers targeted systems in Illinois and Arizona. FBI Director James Comey told lawmakers this week that the agency is looking “very, very hard” at Russian hackers who may try to disrupt the U.S. election.

Last month, Donald Trump, the Republican nominee for president, suggested that he feared the general election “is going to be rigged.”

The FBI has detected a variety of “scanning activities” that are early indications of hacking, Comey told the House Judiciary Committee this week.

The Department of Homeland Security is offering states more comprehensive, on-site risk and vulnerability checks. Only four states have expressed interest in the assessment, and because the election is only weeks away, the department will likely only be able to conduct an assessment of one state before Election Day on Nov. 8, the official said.