Updated at 5:50 p.m. ET
At least three more senators today received suspicious packages in the mail at their local offices, CBS News confirms.
The Hartford office of Sen. Joe Lieberman, I-Conn., received a suspicious package, and as a precautionary measure, his office contacted local authorities and U.S. Capitol Police, Lieberman's spokesperson Whitney Phillips confirms.
Capitol Police spokeswoman Kimberly Schneider confirms that the Burlington office of Sen. Patrick Leahy, D-Vt., also received a suspicious package. The Capitol Police are working with the Burlington Police and the FBI to investigate.
CBS affiliate WCAX reports that hazmat teams were suiting up outside of the Burlington office on Thursday afternoon.
The Philadelphia office of Sen. Pat Toomey, R-Penn., also received a suspicious letter Thursday afternoon, and that police and firefighters responded.
Schneider would not characterize the package received at Leahy's office, but Lieberman's spokesperson said the package sent to the Connecticut senator came from a return address Capitol Police warned congressional offices to watch for.
in the past two days have received "threatening mail containing a suspicious powdery substance," though the messages have been found harmless so far.
The FBI said in a statement it is aware of this situation and responding accordingly.
"We are working with those law enforcement agencies affected to determine if the mailings are related," the statement said. "Even sending a hoax letter is a serious crime that will be thoroughly investigated."
Letters were also sent to several media organizations, including Comedy Central comedians Jon Stewart and Stephen Colbert. The author told the comedians he would send letters to all 100 senators and 10 percent of them would contain "lethal pathogens," an official told CBS News.
The FBI gave corporate security departments a copy of a two-page letter to be on the watch for.
The letters received so far all have had return addresses from Portland, Oregon, and some say they are from an organization listed as "The MIB, LLC." The sender made a list of demands, including an "end to corporate money and 'lobbying,'" an end to corporate "personhood" and a new constitutional convention. All the letters that congressional offices have received so far were mailed around February 18.