US officials security specialists warn that Russian hackers may answer new sanctions by attacking computer networks of US banks and large companies. Officials involved in a White House review of the effects of further penalties on Russia did not respond to questions about whether the study explored the risk of cyberattacks. Even so, two sources said it included revisiting previous classified exercises in which small numbers of computer experts showed they were able to cripple the United States economy in a few days. Cybersecurity specialists consider Russian hackers among the world’s best at infiltrating networks and say evidence exists that they have already inserted malicious software on computers in the US. More