And it's not only the jury's stunning verdict.
When Casey's mother, Cindy Anthony, delivered the bombshell testimony that she was the one who performed the incriminating computer searches on the family computer, "It was annoying," the assistant state attorney tells PEOPLE.
"The main thing that went through my head was, 'Why are you doing this now? Now I have to go through the process of impeaching your testimony.' With the exception of that issue, Cindy had always been pretty honest about things."
As for Casey's father, George Anthony, Ashton says that he was combative during his deposition, but cooperated during the trial. "In the end, George and Cindy came to understand that we were doing what we needed to do," says Ashton. "They respected what we were doing, even if they didn't agree."
Of course, the prosecutor wasn't thrilled with the jury's finding either.
The Surprising Verdict
Sitting at the prosecution table when the verdict was read, all Ashton could say was "wow," before leaving the courtroom in stunned silence.
"We were definitely not expecting what happened," he recalls. "Disappointment is the best way to describe it. We put three years of intense work and sacrifice into the case, and to have it end that way, well, that's frustration."
Although Ashton is a fiery prosecutor who often raises his voice in the courtroom, he maintains that he is not mad at the seven women and five men who acquitted the 25-year-old Florida mom.
"You can't get angry at people who are doing the right thing," he says. "I would love to know how they perceived the evidence. I would love to know their take on things: the lies, the 31 days. It would be fascinating to know what they thought."
As for Casey Anthony, who could be a free woman as early as this week, "The verdict hasn't changed what I think about her case or what she did," says Ashton. "But now I am curious about what will happen to her. Who will hire her? Where will she live? What the heck is she going to do now?"
One thing he doesn't expect is for Anthony to provide any new insights as to how Caylee died. "She's like the boy who cried wolf," says Ashton. "We will never know the truth. The only person who can tell it is Casey, and no one will believe what she says. There are some mysteries that will never be solved."