Merkel – who sparked controversy with her open door policy which allowed more than one million migrants into her nation – admitted people wishing to harm Germany may have crossed the border along with others seeking help.
The chancellor said: “There is no doubt that among so many people who have sought shelter in our country, there were also people who have become the focus of the security authorities.”
She went on to claim Germany “owe it to the many unscrupulous refugees” to take action against “those who despicably abuse our willingness to help”.
Mrs Merkel claimed police needed to do more to stop more attacks
Dortmund explosion: First pictures of the team bus after incident
Tue, April 11, 2017
Click through the gallery for the first snaps of the Dortmund team bus after an 'explosion'
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The Chancellor stood firm in the aftermath of the attack on the Borrusia Dortmund team bus, saying Germans “will never resign ourselves to terror”.
Mrs Merkel said: "We know that we are threatened, like many other countries, and do everything we can to ensure freedom for our citizens.”
But the leader of the central European powerhouse admitted police needed to do more to stop more attacks across the country.
She added: “Unfortunately still a very different level of laws in the individual federal states.
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Merkel sparked controversy with her open door policy
The man suspected of bombing the Borussia Dortmund team bus is an ISIS member
"The federal states should work to achieve an equal level of security, because North Rhine-Westphalia, for example, unfortunately does not practice random checks.
“It would be very useful to carry out these suspicious independent person controls in all the federal states.”
It comes after it was revealed the man suspected of bombing the Borussia Dortmund team bus is an Iraqi-born senior member of Islamic State.
A German judge approved an arrest warrant to keep the 26-year-old Iraqi national – identified only as Abdul Beset A – locked up for allegedly having been a member of the jihadi terror group.
Three explosions on Tuesday injured a player and a police officer
He is believed to have led 10 fighters in Iraq but prosecutors said there was no evidence linking him to the bus attack.
Three explosions on Tuesday injured a player and a police officer.
The bombs exploded as the Borussia Dortmund bus travelled to the club’s stadium for the Champions League match against AS Monaco.
The game was played the next day when Borussia Dortmund lost 2-3.