The holiday data breach at Target appeared to be part of a broad and highly sophisticated international hacking campaign against multiple retailers, according to a report prepared by federal and private investigators that was sent to financial-services companies and retailers. The report offers some of the first details to emerge about the source of the attack that compromised 40 million credit- and debit-card accounts and personal data for 70 million people. It also provided further evidence the attack on Target during peak holiday shopping was part of a concerted effort by skilled hackers. Parts of the malicious computer code used against Target’s credit-card readers had been on the Internet’s black market since last spring and were partly written in Russian, people familiar with the report said. Both details suggest the attack may have ties to organized crime in the former Soviet Union, former U.S. officials said.