Many users of Google’s free e-mail service Gmail were stranded Tuesday afternoon when they logged on to find the service no longer working.

It wasn’t immediately clear what caused Tuesday’s disruption, which led Gmail users to get an “Unable to reach Gmail” error message as their computers tried repeatedly to reconnect to the service. Google said it was investigating.

Complaints began appearing on Twitter around 4 p.m. EDT. In a 4:02 p.m. EDT post on its Apps Status page, Google confirmed that Gmail was suffering an outage.

Google has kept its major outages relatively rare. Gmail went down for more than two hours in February and about the same length of time in August 2008. In May, the search engine was unavailable to many users for about an hour.