Sheriff's officials say there's nothing they can do after outrage greeted the disclosure that Nikolas Cruz, who confessed to killing 17 people at a Florida high school, was allowed to register to vote from his jail cell over the summer.

Cruz, 20, who is charged with 17 counts of murder and 17 counts of attempted murder in the assault on Marjory Stoneman Douglas High School in Parkland on Valentine's Day, registered as a Republican on July 25, listing the address of the Broward County Jail as his residence, according to state voter registration records.

Nikolas Cruz is escorted into court for his arraignment at the Broward County Courthouse in March in connection with the deaths of 17 people at Marjory Stoneman Douglas High School in Parkland, Florida. Amy Beth Bennett / South Florida Sun Sentinel via Getty Images

Cruz's registration came to light on Saturday, when Andrew Pollack, the father of Meadow Pollack, one of the 14 pupils who were killed on Feb. 14, tweeted his anger.

Referring to Cruz by his court case number, Pollack wrote: "I'm sick to my stomach. 18-1958 murdered 17 students & staff, including my daughter Meadow. Yet in July, Broward Sheriff @ScottJIsrael let people into the jail to get him & other animals registered to vote. The Despicable Democrats have no shame. Can't let them steal this election."

Israel was re-elected sheriff in 2016 as a Democrat.

Florida law allows criminal defendants to vote as long as they haven't been convicted. While Cruz has confessed to the killings, he is still awaiting trial.

"Taking away the voting rights of an eligible person awaiting trial is illegal," said Veda Coleman-Wright, a spokeswoman for the sheriff's office, according to NBC affiliate WFLA of Tampa. "BSO serves as a mail courier between the inmates and the Supervisor of Elections office."

In an interview Sunday morning on Fox News Channel's "Fox & Friends," Pollack called Cruz's registration "a dagger in my heart."

"What kills me, too, is my daughter just turned 18," he said. "This would have been her first election. And she was robbed from that."

Pollack, an outspoken supporter of President Donald Trump, continued tweeting throughout Saturday and Sunday, warning that "sleaze ball Democrats" shouldn't be allowed to "steal" a recount of the ballots in the undecided Senate election between Democratic Sen. Bill Nelson and Republican Gov. Rick Scott.

Scott tweeted his thanks to Pollack, adding: "We will not let unethical liberals steal this election!"