Experts urged the journal to cancel publication of the article, one calling it “unwise, unjustified, and dangerous.” Even before publication, a report from a World Health Organization meeting noted that the endeavor “did not require exceptional biochemical knowledge or skills, significant funds or significant time.”

But the study’s lead researcher, David Evans, a virologist at the University of Alberta, said he had alerted several Canadian government authorities to his poxvirus venture, and none had raised an objection.

Many experts agree that it would be very difficult for amateur biologists of any stripe to design a killer virus on their own. But as more hackers trade computer code for the genetic kind, and as their skills become increasingly sophisticated, health security experts fear that the potential for abuse may be growing.

“To unleash something deadly, that could really happen any day now — today,” said Dr. George Church, a researcher at Harvard and a leading synthetic biologist. “The pragmatic people would just engineer drug-resistant anthrax or highly transmissible influenza. Some recipes are online.”

“If they’re willing to inject themselves with hormones to make their muscles bigger, you can imagine they’d be willing to test more powerful things,” he added. “Anyone who does synthetic biology should be under surveillance, and anyone who does it without a license should be suspect.”

Authorities in the United States have been hesitant to undertake actions that could squelch innovation or impinge on intellectual property. The laws that cover biotechnology have not been significantly updated in decades, forcing regulators to rely on outdated frameworks to govern new technologies.