Couldn't tell you - Tradução em português

What is the meaning of "I couldn't tell you" in the following context?
It's part of news from CNN( today) about the two Rutgers University students accused of invading the privacy of fellow student Tyler Clementi, who committed suicide after his sexual encounter with another man was streamed online.

"To know that two intelligent kids could get caught up in something like this is shocking to me," Ardeshna said. "The only rationale I've been able to come up with is that they thought they were being funny -- but I really couldn't tell you.

Thanks in advance,

ailime