Derby say they are "in discussion" with officials from Leeds about what happened.
Police attended Derby's training ground and told Rams boss Frank Lampard to halt training as they dealt with the extraordinary issue.
Lampard revealed that Derby don't intend to lodge a complaint about the matter but added that Leeds also spied on his team's training session before the sides met in August, with Bielsa's team winning that game 4-1. I am responsible for it.
Derby, however, moved to point the finger at Leeds this afternoon, a matter of hours ahead of their visit to Elland Road.
'We had somebody [spy on us] the first game we played against them, which we lost 4-1.
'It's disrupted our build-up to this game, and the unfortunate thing for us is it's disrupted us from our end without doing anything.
'What I will say is, we were training the day before the game on team tactics, team shape, personnel, how we're going to press, how we're going to work off the ball.
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"So I think you look at all three of those games and you think, we can go away from home and play well and almost get a result against Chelsea and obviously win the other two games".
Although the practice of spying on opponents' training sessions may be more common in other countries, Lampard believes that isn't a suitable excuse for Leeds' actions. The man was asked to leave but it wasn't followed up like it was this time'.
I don't care if it's cultural. I have a different point of view on it, but the important thing is what Frank Lampard and Derby think.
"If I'm lucky enough to do well and travel to another country I'll find out what the etiquette is in that country and abide by that".
Who was the "spy" outside Derby's training ground?
"I don't think we need to [investigate], because he's admitted it so it's easily dealt with".
Derby County have very much been the nearly-men of the Championship over the past few years, often falling at the final hurdles keeping them from returning to the Premier League.