BOSTON (Reuters) - A Massachusetts man was sentenced on Friday to five years of probation after admitting he mailed threatening letters filled with a suspicious white powder to several prominent people, including U.S. President Donald Trump’s sons.

FILE PHOTO: Daniel Frisiello, charged with sending a series of threatening letters containing a suspicious powder, including one addressed to U.S. President Donald Trump's eldest son, Donald Trump Jr., appears in a booking photo provided by the Beverly Police Department, March 1, 2018. Beverly Police Department/Handout via REUTERS

Federal prosecutors in Boston had sought three years in prison for Daniel Frisiello, 25, of Beverly, Massachusetts, whose hoax letters included one in February 2018 addressed to Donald Trump Jr. Trump’s then-wife, Vanessa, opened the letter and was taken to the hospital.

Investigators later determined the powder sent to Trump’s eldest son’s home was harmless, as was the substance contained in an earlier letter Frisiello sent during the 2016 presidential campaign to the Republican president’s son Eric Trump.

U.S. District Court Judge Nathaniel Gorton in Boston sentenced Frisiello to five years of probation, with one year to be served in home detention with location monitoring, and restitution.

During the period of probation, Frisiello is prohibited from contacting victims in the case, sending letters through the mail, accessing the internet, and possessing and accessing computers.

He pleaded guilty in October to charges related to 13 threatening letters he mailed from 2015 to 2018, six of which contained a white powder.

U.S. authorities have been on alert for mail containing white powder since 2001, when envelopes laced with anthrax were sent to media outlets and U.S. lawmakers, killing five people.

Other letters containing a white powder were mailed to Democratic U.S. Senator Debbie Stabenow of Michigan and Antonio Sabato Jr, an actor who in 2018 was a Republican candidate in California for the U.S. House of Representatives.

Frisiello also sent threatening letters to prosecutors and police chiefs overseeing high-profile criminal cases he objected to and a Massachusetts company that fired one of his relatives, prosecutors said.

The letter he sent to Trump Jr’s address in Manhattan read in part, “You are an awful, awful person, I am surprised that your father lets you speak on TV,” prosecutors said.

Reviews of social media posts led prosecutors to view Frisiello as a suspect and a search of his trash revealed a computer printout referencing two of the people who received the threatening letters, prosecutors said.

His lawyers sought five years of probation, saying in court papers that Frisiello had developmental delays and is autistic, facts that provide “context to understand the genesis of his crimes and to gauge the degree of his moral culpability.”