Sen. Barbara Boxer (D-CA) organized the 16 senators to sign the private letter, Democratic aides told ABC News.
Boxer did not comment to ABC about the letter, but reiterated her support for Clinton.
"I have repeatedly said -- publicly, privately and in writing -- that Hillary Clinton should run for president in 2016," Boxer said in a statement to ABC news. "I can only speak for myself. I'll leave it to my colleagues to describe their views."
Boxer's support for Clinton comes as less of a surprise than the support of senators like Elizabeth Warren (D-MA) and Kirsten Gillibrand (D-NY), whose names have been discussed as possible candidates in 2016 as well.
The senators reportedly sent the letter to Clinton in February.
Source tells me the letter from female Democratic senators in support of Hillary Clinton was sent back in February— Rebecca Berg (@rebeccagberg) October 30, 2013