Tunisia says suicide bomber behind bus attack that killed 13

By Tarek Amara TUNIS (Reuters) - Tunisian security officials said on Wednesday a suicide bomber carried out the attack on a presidential guard bus, killing at least 13 and forcing the government to impose a nationwide state of emergency. Tuesday's blast on a main boulevard in the capital underscored Tunisia's vulnerability to Islamist militancy following the gun assaults on a Sousse hotel in June and the Bardo Museum in Tunis in March, both claimed by Islamic State. It was also the first suicide