The FBI has arrested a Massachusetts man believed to have sent Stanford law professor Michele Dauber a threatening letter containing a suspicious white power last month.

Two rooms in the law school campus were closed temporarily on Feb. 14 while a hazardous materials team tested the substance, which they determined posed no threat.

Dauber was targeted for her role as the chair of the Recall Persky Campaign, which seeks to oust Santa Clara Superior Court Judge Aaron Persky for what critics say was a lenient six-month jail sentence for Brock Turner—a former Stanford athlete who was convicted of sexually assaulting an unconscious woman outside a fraternity party.

According to the U.S. Attorney's Office for the District of Massachusetts, 24-year-old Daniel Frisiello, of Beverly, Massachusetts, sent threatening letters with suspicious powder to five people, including Dauber. Among Frisiello's other alleged targets is Donald Trump Jr., whose wife opened the letter containing powder. (That letter called Trump an “awful person” and said, “You make the family idiot, Eric, look smart.)

“Beyond terrifying the victims, these incidents caused law enforcement agencies around the country to spend time and money deploying emergency response units,” said U.S. Attorney Andrew Lelling in an announcement of the arrest.

Dauber said she is relieved by the arrest. “Our campaign is not going to be intimidated by threats and personal attacks,” she said. “We are going to continue to stand with survivors of sexual assault and on June 5, the voters will recall Judge Persky.”

The threatening note to Dauber read, “Since you are going to disrobe Persky, I am going to treat you like 'Emily Doe'. Let's see what kind of sentencing I get for being a rich white male.”

According to authorities, Frisiello also sent Dauber a so-called glitter bomb—a letter filled with glitter that spills on the recipient when opened. Investigators traced financial records that showed he had paid an online company to have the glitter bomb sent to Dauber. The FBI also looked through Frisiello's trash and found what appeared to be remnants of the cut-out messages he is accused of sending.

Frisiello was arrested on Thursday. He has been charged with five counts of mailing a threat to injure the person of another and five counts of false information and hoaxes.

His attorney, Scott Gleason, did not immediately respond to a request for comment Friday. A preliminary detention hearing is scheduled for Monday.