Iran has restored blocks on Facebook and Twitter after a "technical glitch" briefly removed filters from the social networks overnight.
Iranians gained direct access to the sites for the first time in four years because of a fault, an official said, denying suggestions the government had lifted a ban in place since anti-government protests in 2009, some of which were organised on social media.
Newly elected President Hassan Rouhani has pledged greater engagement with the West and a new openness in Iran.
It has been suggested the "glitch" could point to internal power struggles between groups seeking to reopen Facebook and other social networking sites and hard-liners in the establishment, who remain in control of Internet access.
But Abdolsamad Khoramabadi, secretary of a state committee tasked with monitoring and filtering sites, said problems with some Iranian Internet service providers (ISPs) had allowed the access, and the government was investigating.
"The lack of a filter on Facebook last night (Monday) was apparently due to technical problems and the technological committee is investigating this issue," Mr Khoramabadi was quoted by Mehr news agency as saying.
"We are investigating to see which of these companies has done this."
Since Mr Rouhani took office last month, there has been a muted thaw in the restrictions around social media.
Officials, including Foreign Minister Javad Zarif, have created Facebook and Twitter profiles, raising hopes among some Iranians that the sites would soon be unblocked for them.
Now, many use proxy servers to trick the system into believing they live elsewhere to access their social media accounts.
Arash Tajik, an IT administrator in Tehran, said he thought the blip, which meant he could access Facebook without a proxy server at his office on Monday evening but not from his home on Tuesday, might be a test.
He said: "They are testing what will happen if they remove the filter, and whether they can control the situation or not."
Mr Rouhani has pledged to relax political and social restrictions in Iran, which were ramped up after the disputed election in 2009 sparked protests that were often organised via social media.
Several dissidents and activists have been put in jail or forced to leave Iran since.
However, any move to ease control will first have to be approved by the ruling establishment of conservative clerics and security officials, including Supreme Leader Ayatollah Ali Khamenei.
- Related Stories
- Iran To Assign Email Addresses To Citizens
- Iran Minister's 'Happy New Year' Tweet To Jews