(Andrea Bruce for The Washington Post)

U.S. officials know that Tehran is using communications equipment — some perhaps acquired from American firms — to monitor and, in some cases, interfere with Iranians’ use of the Internet and mobile phones. And yet it’s proven difficult to do anything about it.

An economic sanctions bill against Iran, signed into law by President Obama last July, established a procurement ban on any entities found to have exported sensitive technology to the Islamic Republic that could be used to stifle dissent.

Since then, the Iranian government has continued suppressing dissent online. But, according to the Government Accountability Office, the State Department has not identified a single individual or firm to subject to the procurement ban. Neither has the GAO, which used “credible open source information” to go looking on its own.

The explanation, the GAO said in a report released this week, may have something to do with the proprietary nature of the communications industry — and its tendency to limit information about its own dealings. Or it might just be too hard to distinguish between technologies exported to Iran for reasonable purposes (“necessary and acceptable” filtering) and technologies exported to Iran for more sinister reasons.

Lastly, the problem might just be that Iran is starting to produce very good snooping equipment on its own, making it less dependent on foreign firms.

Whatever the reason, Iranian democracy activists continue to suffer the consequences. In its report on Internet freedom this spring, a Washington pro-democracy group concluded that, since the disputed presidential elections in 2009, Iran has used sophisticated technologies to wage a campaign against the free flow of information online.

The group, Freedom House, said that the campaign has gone “well beyond simple content filtering” to include, among other things, “deliberately slowing down Internet speeds at critical times to make basic online activities difficult.”

In the group’s ranking of Internet freedom in individual countries, Iran placed dead last, behind Burma, Cuba and China. North Korea, among other countries, was not surveyed.