Please answer a question about the following article about FC Barcelona:

Despite being the favourites and starting strongly, Barcelona finished the 2006–07 season without trophies. A pre-season US tour was later blamed for a string of injuries to key players, including leading scorer Eto'o and rising star Lionel Messi. There was open feuding as Eto'o publicly criticized coach Frank Rijkaard and Ronaldinho. Ronaldinho also admitted that a lack of fitness affected his form. In La Liga, Barcelona were in first place for much of the season, but inconsistency in the New Year saw Real Madrid overtake them to become champions. Barcelona advanced to the semi-finals of the Copa del Rey, winning the first leg against Getafe 5–2, with a goal from Messi bringing comparison to Diego Maradona's goal of the century, but then lost the second leg 4–0. They took part in the 2006 FIFA Club World Cup, but were beaten by a late goal in the final against Brazilian side Internacional. In the Champions League, Barcelona were knocked out of the competition in the last 16 by eventual runners-up Liverpool on away goals.

What event was blamed for the injuries to Barcelona stars?
US tour