SAN FRANCISCO — The Heartbleed bug that made news last week drew attention to one of the least understood elements of the Internet: Much of the invisible backbone of websites from Google to Amazon to the Federal Bureau of Investigation was built by volunteer programmers in what is known as the open-source community.

Heartbleed originated in this community, in which these volunteers, connected over the Internet, work together to build free software, to maintain and improve it and to look for bugs. Ideally, they check one another’s work in a peer review system similar to that found in science, or at least on the nonprofit Wikipedia, where motivated volunteers regularly add new information and fix others’ mistakes.

This process, advocates say, ensures trustworthy computer code.

But since the Heartbleed flaw got through, causing fears — as yet unproved — of widespread damage, members of that world are questioning whether the system is working the way it should.

“This bug was introduced two years ago, and yet nobody took the time to notice it,” said Steven M. Bellovin, a computer science professor at Columbia University. “Everybody’s job is not anybody’s job.”