The websites of at least three German airports were disrupted on Thursday, a day after a major IT failure at Lufthansa left thousands of passengers stranded at Frankfurt airport.
Among the airports affected were Dusseldorf, Nuremberg and Dortmund, but sites for Germany's biggest airports, in Frankfurt, Munich and Berlin, were operating normally.
Focus Online, which initially reported the outage, gave no reason for the problem.
"We are troubleshooting", said a spokesperson for Dortmund airport, adding it was unlikely that the failure was due to a regular overload. "There is reason to suspect it could be a hacker attack," said the spokesperson.
Spiegel Online reported that the problems could have been caused by a DDos attack, where high volumes of internet traffic are directed to targeted servers in a relatively unsophisticated bid by so-called "hacktivists" to knock them offline.
