BOSTON/WASHINGTON -- Target Corp and Neiman Marcus are not the only U.S. retailers whose networks were breached over the holiday shopping season late 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 attacks.

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

Law enforcement sources have said they suspect the ring leaders are from Eastern Europe, which is where most big cyber crime cases have been hatched over the past decade.

Only one well-known retailer, Neiman Marcus, has said that they too have been victim of a cyber attack since Target's December 19 disclosure that some 40 million payment card numbers had been stolen in a cyber attack. On Friday, Target said an investigation found that hackers stole the personal information of at least 70 million customers, including names, mailing addresses, telephone numbers and email addresses.

Neiman Marcus said it was not sure if the breach was related to the Target incident.

Most states have laws that require companies to contact customers when certain types of personal information is compromised. In many cases the task of notification falls on the credit card issuer.

Merchants are required to report breaches when certain types of personal information, including social security numbers are compromised. It was not immediately clear if that was the case with the retailers who were attacked around the same time as Target.

The Secret Service and Department of Justice, which are investigating the Target breach, declined comment.

Related: