The e-mails arrived like poison darts from cyberspace.

Some went to the Chertoff Group, a national security consultant in Washington. Others targeted intelligence contractors, gas pipeline executives and industrial-control security specialists. Each note came with the personal touches of a friend or colleague.

“Attach[ed] is a quote for the Social Media training we discussed,” said one note sent on July 3 to the vice president of EnergySec, a federally funded group in Oregon that focuses on the cybersecurity of the nation’s power grid.

But like much of the digital universe, the e-mails were not what they seemed. They were cyberweapons, part of a devastating kind of attack known as “social engineering.”