WASHINGTON—President Barack Obama said the shooter who killed 49 people and injured 53 others at an Orlando gay nightclub apparently was self-radicalized and “an example of homegrown extremism.”

The president noted the man police have identified as the gunman, Omar S. Mateen, purchased a gun legally and was inspired by extremist information disseminated over the internet. Mateen pledged allegiance to Islamic State “at the last minute,” Mr. Obama said.

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