A federal judge sentenced an Iowa man to six years in federal prison for sending threatening tweets to the state’s Republican junior senator, Joni Ernst.

Joseph Hilton Dierks, 34, from Waterloo, received the sentence Wednesday after a jury convicted him on three counts of sending threatening communications, according to the U.S. Attorney’s Office for the Northern District of Iowa.

“The safety of elected officials is something that cannot be taken lightly,” U.S. Attorney Peter E. Deegan, Jr., said in a news release. “This sentencing sends the message that those who attempt to intimidate or threaten to harm elected officials will be held accountable for their actions.”

Dierks is now in the U.S. Marshal’s custody awaiting transportation to federal prison.

Dierks first caught the attention of law enforcement in August of last year, when he started tweeting threatening messages to Ernst’s Twitter account. U.S. Capitol Police contacted the Waterloo Police Department, which sent an officer to Dierks’ house.

Police took extra precautions in handling the threats because Ernst was in Iowa at the time, according to the news release. Ernst has represented the state in the Senate since 2015.

When an officer knocked on Dierks’ door, he promised to “tone it down.” But about a day later, officers found him back on Twitter firing off threats.

The FBI investigated the case with support from Capitol and Waterloo police.

The length of Dierks’ sentence was  based in part on U.S. District Court Judge Linda Reade’s judgment that he was at an “extreme risk” to repeat the offense. Dierks said in a recorded jailhouse telephone call with his mother that he had “no intention of following the Court’s orders when released from prison.”

Dierks also had a record of crime and violent tendencies. He was previously convicted for harassing a Waterloo police officer and carrying weapons. He also filmed himself threatening an unwitting neighbor with a knife.

In January, a Nebraska man pled guilty to threatening Ernst last July and awaits sentencing.

Robert W. Simet, 64, told employees at a motorcycle shop near the Nebraska-Iowa state line that he might kill Ernst at a speech she was scheduled to deliver there. He said he believed Ernst was in involved in a conspiracy with Islamic State terrorists.