One of al-Qaeda’s primary Web forums was back online Wednesday after a lengthy blackout that analysts said appeared to have the hallmarks of a cyberattack.

The outages extended to at least five other sites associated with al-Qaeda, most of which have been dark for at least a week and a half — the longest sustained blackout for such forums.

Shumukh al-Islam, regarded as one of the terrorist network’s two main sites, went down March 22 and stayed offline for most of the past 13 days. When it returned Wednesday, it was accompanied by a message stating that “the enemies of Allah . . . attempted to target” the forum “with a failed, miserable campaign.”

The message, posted by someone with administrative access to the site, said that in return, “dozens of forums instead of just one” would arise, according to a translation by Flashpoint Global Partners, which tracks jihadi Web sites.

No one has claimed responsibility for the outages, and U.S. officials have said that no U.S. agency, including the military’s Cyber Command, was involved.

Five other sites, including al-Fida’, al-Qaeda’s other main forum, were still offline Wednesday.

Some analysts have speculated that the administrators of the sites might have taken them down if they suspected that the forums had been infiltrated by foreign spies. But the message on Shumukh suggests that the outage was “at least partly an aggressive attack,” said Evan Kohlmann, senior partner at Flashpoint.

He said that if Shumukh closes, another site probably will take its place. He noted that Shumukh became a top-tier forum only after a separate site, al-Fallujah, was shuttered in 2010.

“The long-term consequences come only if all the forums are taken out in such a way that there is no clear line of succession,” Kohlmann said. “Then people don’t know where to turn” for jihadi messages.

The SITE Intelligence Group, a private organization that also tracks jihadi sites, agreed. Within the past five years, the top-tier forums have all seen downtime — sometimes briefly, sometimes permanently, it noted in a statement this week.

“The online jihadist community will not be dismantled simply by taking two Websites offline,” the statement said. “The decentralized and redundant nature of the Internet ensures that it will be extremely difficult to disrupt the jihadists’ online networks.”

Repeated attacks probably will force the jihadists to improve their security, making it more difficult for any intelligence agency trying to hack any network, Kohlmann said. Already, he said, administrators on other sites are advising users trying to log on to Shumukh to use a secure, encrypted connection.