"A man was found at the perimeter fence of Moor Farm and was spoken to", they said on Twitter.
Asked about the "spy" before facing Derby, Bielsa told Sky Sports: "It's true it was someone from Leeds United". I always try to find information and and to know a little bit more about what the opponent is doing.
Tottenham manager and fellow Argentine Mauricio Pochettino was one of the few to defend Bielsa, whom he played under at Newell's Old Boys in their native country, claiming the trick was widespread among the nation's coaches.
The Championship leaders have now stated their intention to speak with Bielsa, who conceded he personally sent the man in question without notifying the club, regarding his remarks.
Leeds says owner Andrea Radrizzani apologized in person to Derby owner Mel Morris.
'We will make no further comment on this matter'.
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Pochettino also appeared to hint that his side may need reinforcements after Harry Kane limped off with an injury at the end of the game.
"For me it's enough that Frank Lampard and Derby County felt that it was not the right thing to do".
An official complaint could be made to the EFL or the Football Association but the EFL said today: "We are aware of the reports in relation to an incident at the training ground of Derby County".
"It doesn't matter if it's legal or illegal, or right or wrong".
"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".
Leeds went on to win the fixture 2-0, courtesy of goals from Kemar Roofe and Jack Harrison as they extended their lead at the summit of the Championship table.
The Whites took full advantage of their opportunity to pile on the pressure on their promotion rivals, ahead of their matches on Saturday, and they will watch on with interest to see how those clubs respond.
"I would accept any sanction that the club takes against me, I would accept any sanction that the federation takes against me and I would accept the judgement of Derby County and Frank Lampard".