Hoffenheim's former Liverpool forward Ryan Babel has again landed himself in hot water with football officials after another outburst on Twitter.
The 25-year-old is being investigated by the German Football Federation (DFB) after posting, "I don't know, I think the referee was on drugs," on his official Twitter account after being sent off over the weekend in the Bundesliga.
The Netherlands international was shown two yellow cards in the side's 3-1 defeat at Hertha Berlin, after a clash with the host's captain, Levan Kobiashvili, and made the remark about referee Thorsten Kinhoefer.
The DFB's disciplinary committee demanded a statement from Babel, who was also in trouble for a Twitter post while playing for Liverpool.
In January 2011, Babel was fined nearly $16000 after admitting a charge of improper conduct by the Football Association for posting a link to a mocked-up picture of referee Howard Webb wearing a Manchester United shirt.
In addition to sending the link to his followers, Babel wrote: "And they call him one of the best referees. That's a joke," after Webb sent off Liverpool captain Steven Gerrard in a 1-0 FA Cup defeat to United.
Babel joined Hoffenheim from Liverpool later that month, but this latest episode will further strain relations with coach Markus Babbel, who dubbed the winger's actions "stupid" after he was sent off in Berlin.
Eintracht Frankfurt goalkeeper Oka Nikolov will end a 19-year association with the Bundesliga club to continue his career in Major League Soccer.