Chelsea have begun an investigation after a smoke grenade went off at the club's Surrey training ground.
Players and officials from the Blues' reserve, youth and academy teams had to evacuate the building in Cobham because of thick smoke, according to reports.
"I can confirm an investigation is under way. It is an internal matter," a Chelsea spokesman said.
The club did not give any details about the investigation, or if any players were suspected of involvement.
There have been two other incidents at the training ground in the past 13 months.
In December 2011 it was reported that cleaning staff discovered a knife in a dressing room at the complex, although this was believed to have belonged to a building contractor on the site.
And in February 2011, Chelsea defender Ashley Cole accidentally shot and wounded a work experience student with an air rifle at the training ground.
It is believed Cole did not know the rifle was loaded and that he also apologised to the student, who did not require hospital treatment.