A California man hacked and defaced websites for a West Point counterterrorism research center and a New York City government agency during a campaign of vandalism, federal prosecutors said on Thursday.

A criminal complaint accuses Billy Ribeiro Anderson with hacking the two sites using the online pseudonym "Alfabeto Virtual." It says he modified them to display "Hacked by Alfabeto Virtual."

The FBI arrested the 41-year-old Anderson early Thursday at his home in Torrance, California. He was to make a court appearance later in the day in Los Angeles.

There was no lawyer on record who could comment for the defendant.

Before his arrest, Anderson had used his computer skills to vandalize U.S. military, government and business websites more than 11,000 times since 2015, authorities said.

"Among other possible effects, website defacements can disrupt an organization's operations and damage its credibility," said William Sweeney, head of the FBI's New York office.

The alleged hacker struck the website for the New York City Comptroller in 2015 and, along with his pseudonym, plastered #FREEPALESTINE" and "#FREEGAZA" on the site. About a year later, he defaced the site for the Combating Terrorism Center at West Point.

Both West Point and the city agency had to spend thousands of dollars to correct the security breach, authorities said.