In addition to the Islamic State flag, visitors to the site were met by the "National Anthem" music which is used by ISIS in several of its videos, including those showing executions of hostages.
Newspaper Dagens Nyheter reports that the hacked Expressen page appears to be identical to the one uploaded on to the Västerås Airport website on Saturday night.
Shortly after 9am on Sunday, Expressen had taken away the ISIS symbol and sound file. Expressen's editor, Thomas Mattsson, took up the hacking in his Sunday blog and apologised to his readership for any inconvenience.
"The site is hacked by someone purporting to represent the terror group Islamic State, but of course it is not yet known who is responsible for the illegal hacking. We have pointed traffic to Expressen's news site, the operating organization is working with the error and the police have been informed."
On Sunday, Expressen also closed down several of its blogs as a precaution.
"I regret that anyone who tried to disrupt our journalistic activities have succeeded, but have great confidence in Expressen's IT organization. Our news has not been affected, we have consistently reported what is occuring in the world and, in fact, on our own site."
The acting manager of Västerås Airport was also angry at the illegal hacking of its own website the night before.
"I am very upset and concerned," Helena Burén said to Västmanland's county newspaper.
"The police are informed and this hacking is reported as an illegal hacking. Yesterday it was Västerås Airport that was hit and today it is a big media site, this is bad thing to do but also a reminder of the vulnerability of society and the increasing need for IT security."