Police in New York are also investigating a letter containing white powder that was sent to the Manhattan office of former New York City mayor Rudy Giuliani.
An unnamed federal law enforcement official said powder from one envelope tested positive for baking soda, according to Katie Zezima The Associated Press.
Law enforcement officials described the investigations as "routine" and said the incidents are not considered a threat.
The white powder was delivered to two hotels in Carlstadt, N.J., and another in East Rutherford -- the home of MetLife Stadium, where the Denver Broncos will play the Seattle Seahawks for the NFL title on Sunday -- said Jeanne Baratta, chief of staff at the Bergen County Executive's Office.
CNN is reporting a total of six letters have been reported in New Jersey.
The letter found at the office of Guiliani was opened in the mail room.