The gunman at Arapahoe High School in Colorado was looking for a specific teacher when he was confronted by a student, local media has reported.
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The lender, which is the worst affected by the PPI mis-selling scandal, said it has also set aside £150m for other conduct issues.
Vodafone has been fined £4.6 million by the industry regulator for "serious and sustained" breaches of consumer protection rules.