SAN FRANCISCO — The largest online black market for drugs, AlphaBay, has been down for nearly two days, raising questions about whether it was seized by law enforcement authorities or taken down in a swindle.

The administrators of AlphaBay have posted on the social network Reddit that they are doing maintenance and that the site should be up again soon. But those messages were posted long after AlphaBay went down on Tuesday, leading to rampant speculation among its users.

The site is one of many so-called dark net markets that came into existence after American authorities took down the first and most famous such market, the Silk Road, in 2013.

AlphaBay has come under scrutiny most recently because of the many vendors who use it to sell the deadly synthetic opioids at the center of the nation’s overdose crisis. The site is also used as a marketplace for stolen credit cards and other financial data.