Zimmer said she was interviewed by federal prosecutors about her allegations for "several hours" on Monday and gave them pages from her journal that supported them. Kim Guadagno, Christie's Lieutenant-Governor, described the claims as "offensive" yesterday.
"I deny wholeheartedly those allegations," she said.
The prosecutors were already looking into whether laws were broken when Christie aides inflicted traffic gridlock on the town of another Democratic mayor who had refused to support him.
The governor, who previously insisted his team had nothing to do with road closures that caused the gridlock, said he was "embarrassed and humiliated" and fired a senior aide at the centre of the scandal.
Christie rejected claims that he must have known what his advisers were doing.
He likened himself to the chief executive of JP Morgan, who said he was blindsided by the discovery that a trader known as the London Whale lost the bank billions of pounds in 2012.
Amid indications that the scandal has dented his status as the early front-runner to lead the Republicans' bid to reclaim the White House, Christie stressed: "I will learn things from this. I don't know exactly what it is yet ... but when I get the whole story and really try to understand what's going on here, I know I'm going to learn things."