Iranians had a few hours access to Facebook and Twitter before a Web firewall went back up by Tuesday and Tehran scotched talk of new Internet freedoms by blaming a technical glitch for the brief opening of access.
Late on Monday, several people in Iran found they could log in to their accounts on the U.S.-based social media sites without using techniques to circumvent blocks on Twitter and Facebook that the state imposed four years ago, during a clampdown on the biggest protests since the Islamic revolution.
That prompted speculation that it might herald a broader easing of censorship under President Hassan Rouhani; last month, he succeeded Mahmoud Ahmadinejad, whose re-election in 2009 sparked the demonstrations, in which social media played a part.
However, access was being blocked again on Tuesday and an official involved in controlling Internet usage said the brief lifting of the embargo at some Iranian Internet service providers was probably caused by a technical malfunction.
"The lack of a filter on Facebook last night was apparently due to technical problems and the technological committee is investigating this issue," Abdolsamad Khoramabadi, secretary of a state panel that filters sites, told Iran's Mehr news agency.
Service providers were being investigated, he added.
International executives at Facebook and Twitter had no immediate comment on the development.
One Iran expert based abroad said controls had briefly been removed across a very wide range of sites, including online pornography, supporting the view that it was a glitch.
Another, however, said Rouhani's new administration could be considering easing restrictions on sites that remain popular among Iranians able to get around the domestic firewall - and which senior government figures, and even Iran's clerical Supreme Leader, have themselves used to convey their messages.
"I strongly believe it was a technical glitch because all Web sites that support SSL were available in the country last night, even porn websites," said Amin Sabeti, a British-based expert on the Internet in Iran. SSL is a Web security tool.
Sabeti added: "Iran has invested millions of dollars for its filtering system and it is clear that the regime will not give up Internet censorship very easily."
Nonetheless, there have been signs in that direction. Rouhani, a moderately reformist cleric, pledged to relax some social controls during his campaign for June's election. New, U.S.-educated foreign minister,Mohammad Javad Zarif, has Facebook and Twitter profiles and has engaged with other users.
Even Supreme Leader Ayatollah Ali Khamenei seems to have taken to global social media to publicise ideas.
Siavush Randjbar-Daemi, who lectures on Iran at England's Manchester University, said at least partially unblocking sites like Facebook would recognise its popularity. A ban on video recorders, he noted, was lifted in the 1990s when the devices had already become very widely used, despite being outlawed.
"Keeping them illegal became senseless," he said.
Many Iranians use proxy servers to trick systems into believing they are outside Iran to access foreign social media.
Randjbar-Daemi said the authorities might experiment with ways of allowing most access to the likes of Facebook and Twitter, as they do with the search site Google, while using technology to block certain kinds of activity.
"I think we could be seeing a partial unblocking of Facebook, along the lines of Google, in which some search results are filtered but others are not," he said. "Rouhani would also score clear kudos within his supporters in this way."
Arash Tajik, an IT administrator in Tehran, said he believed that the brief opening might have been part of an experiment by the authorities: "They are testing what will happen if they remove the filter and whether they can control the situation or not," said Tajik, who accessed Facebook unfiltered on Monday.
Like Tajik, another Internet user in Tehran called Hossein, said he could not use Facebook without aproxy server on Tuesday. Hamed, a 32-year-old journalist and teacher, said he too found Facebook and Twitter blocked on Tuesday, said Internet providers could face penalties if the opening was unauthorized.
Iran has accused Israel and the United States of cyberwar against its computer systems in the past, notably involving its energy and nuclear facilities. It did not link this week's brief failure of its social media firewall to hacking activity.
Though Rouhani has spoken of reform, any move to ease controls will need approval from the ruling establishment of conservative clerics and security officials, including Khamenei.
Facebook and Twitter became widely accessible to Iranian users on Monday for the first time since 2009, when the services were blocked in the midst of widespread protests against former president Mahmoud Ahmadinejad, according to reports emerging from the country.
Reporters in Tehran for the New York Times and the Washington Post both said on Twitter that they could access the service freely on Monday.
Jillian York, the director for international freedom of expression at the Electronic Frontier Foundation, a civil liberties organization, said she had received multiple reports from citizens using several different Iranian Internet service providers confirming that the bans appeared to have been lifted.
"Some other blocked sites are reporting themselves unblocked - the National Iranian American council is reporting themselves unblocked as well," York said, referring to a U.S.-based nonprofit group.
Iranian authorities blacked out Facebook and Twitter in the summer of 2009, when Ahmadinejad's disputed re-election victory sparked massive protests that gained momentum with the help of organizers using social media.
The administration of the new president, the moderate cleric Hassan Rouhani who took office last month, has signaled - on social media, no less - that it will adopt a much different tack from that of its hardline predecessor.
Marking the Jewish New Year earlier this month, Rouhani and Foreign Minister Javad Zarifwrote on their Twitter accounts to wish Jews a happy Rosh Hashanah.
No Iranian government statement about loosening the restrictions was reported on Monday.