Security experts at Symantec, which in the past has accurately identified attacks mounted by the United States, Israel and North Korea, found early versions of the ransomware, called WannaCry, that used tools that were also deployed against Sony Pictures Entertainment, the Bangladesh central bank last year and Polish banks in February. American officials said Monday that they had seen the same similarities.

All of those attacks were ultimately linked to North Korea; President Barack Obama formally charged the North in late 2014 with destroying computers at Sony in retaliation for a comedy, “The Interview,” that envisioned a C.I.A. plot to kill Kim Jong-un, the country’s leader.

The computer code used in the ransomware bore some striking similarities to the code used in those three attacks. That code has not been widely used, and has been seen only in attacks by North Korean-linked hackers. Researchers at Google and Kaspersky, a Moscow-based cybersecurity firm, confirmed the coding similarities.