The Islamic State suicide bomber who killed 10 German tourists in the heart of Istanbul registered here as a Syrian refugee without setting off security alerts—even though his brother had blown himself up in Syria, Turkish officials said. The revelation highlighted concerns that extremists are using the migrant crisis to cross borders and carry out terrorist attacks.

Turkish officials identified the Istanbul bomber as Nabil Fadli, a Syrian born in 1988, who was fingerprinted in Turkey last week while registering as a refugee...