Leeds Argentine manager Marcelo Bielsa admitted last month that his staff had watched all the club's opponents in training this season.
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Manchester City, Brighton and Millwall all ran out winners in Saturday's FA Cup fifth-round tie action.
Brighton and Hove Albion beat Derby County 2-1 in FA Cup fifth-round despite Ashley Cole goal for visitors.
Leeds manager Marcelo Bielsa admitted last month that his coaching staff had watched all of their opponents in training this season.
Championship side Derby County beat Accrington Stanley of League One 1-0 in a fiercely fought English FA Cup fourth-round tie.
Ashley Cole, who left left LA Galaxy in November and has been a free agent since, has returned to English football after a four and a half years
Marcelo Bielsa didn't have to respond to "spying" accusations, but in the end he did it as only he knew how: with a 70-minute lesson for the media.
Southampton appeared set for an away tie at Accrington Stanley, Derby hit back to level at 2-2 before netting all five penalties in the shootout.
The English Football League has launched a formal investigation after Derby complained that Leeds boss Marcelo Bielsa sent a spy to training.
It's only natural that Marcelo Bielsa would try to glean as much info as possible about rivals -- this type of 'spying' is common in South America.
Marcelo Bielsa has been reminded of his responsibilities after he sent a spy to watch Derby County's training session this week.
"I have to accept that in this country it's not something that is common, but it doesn't make me a person who cheats," Bielsa said of the incident.
"It's true, there was someone from Leeds there. The responsibility for this incident is mine," Bielsa said, adding he apologised to Frank Lampard.
On-loan midfielder Mason Mount was one of Derby's most impressive performers in Carabao Cup defeat at parent club Chelsea.
Frank Lampard was honoured for his past by Chelsea fans and for his present by the young Derby team whose performance did him such credit.
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