The US toy maker claims it has obtained drawings of the original Bratz line-up made by former employee Carter Bryant during his Mattel employment in either 1999 or 2000.
These latest moves, reported in the Wall Street Journal, are the latest in a legal fight over the successful doll that now competes with Barbie in all major territories.
In 2004, Mattel sued Bryant, charging that he secretly worked for a competitor while still employed by Mattel and that he misappropriated intellectually property.
The company asked a federal judge to allow it to amend the lawsuit based on the new information that emerged this week, and also filed for copyright protection on a number of the drawings.
Bratz manufacturer MGA, meanwhile, has pulled no punches in its reply.
It accused Mattel of seeking "revenge on a few people who have chosen to work for a better, more innovative toy company after realising that Mattel has lost touch with its consumers and is hopelessly caught in a downward spiral resulting from mismanagement".