The incident was reported first on Twitter.
The first tweet was a request for more information, along with an image from the scene of emergency vehicles on site.
@miquelmartin (Heidelberg, Germany): Big mess in front of the office in #Heidelberg, anyone has an idea of what's going on? http://twitpic.com/1yff0
Within minutes, additional tweets added context, several minutes before local media reported the story.
@kirscheplotzer (Mannheim, Germany): Schwere Explosion in #heidelberg am Römerkreisel. Vier Schwerverletzte, noch keine genauen Angaben zur Ursache. (Severe explosion in Romans #heidelberg at roundabout. Four seriously injured, yet no precise information on the cause.)
@BlaM4c (Heidelberg, Germany): Der zweite Rettungshubschrauber (siehe http://twitpic.com/1yfrl) ist auch in Richtung Ludwigshafen unterwegs. #Heidelberg #Explosion (The second rescue helicopter (see http://twitpic.com/1yfrl) is also in the direction of Ludwigshafen on the road. #Heidelberg #explosion)
@miquelmartin later posted additional pictures from the scene to his Flickr account and tweeted a link to the site.
@miquelmartin (Heidelberg, Germany): Some pics of the gas explosion on Roemerstr 17 (Romerkreis in #Heidelberg http://is.gd/mxpp #accident #emergency
MorgenWeb.de later reported that the accident was not caused by a gas explosion or leak as initial reports suggested, though the cause was still not clear, according to police.