Factor XIII is also called as fibrin stabilizing factor. It is activated by thrombin. Activated factor XIII strengthens the fibrin reticulum. The fibrin monomer molecules are initially held together by weak non covalent hydrogen bonding, and the newly formed fibers are not cross linked with each other. Therefore as a result, the clot is weak and can be broken apa easily. The activated factor XIII acts as an enzyme to cause covalent bonds between more and more fibrin monomer molecules as well as multiple cross-linkages between adjacent fibrin fibers, adding strength to the fibrin mesh work. Ref: Competition Science Vision, Feb 2003, Pages 1628-9 ; Text Book of Medical Physiology By Guyton and Hall, 10th Edition, Page 422