Target Corp and Neiman Marcus are not the only U.S. retailers whose networks were breached over the holiday shopping season last year, according to sources familiar with attacks on other merchants that have yet to be publicly disclosed. Smaller breaches on at least three other well-known U.S. retailers took place and were conducted using similar techniques as the one on Target, according to the people familiar with the attacks. Those breaches have yet to come to light. Also, similar breaches may have occurred earlier last year. The sources said that they involved retailers with outlets in malls, but declined to elaborate. They also said that while they suspect the perpetrators may be the same as those who launched the Target attack, they cannot be sure because they are still trying to find the culprits behind all of the security breaches.