SAN'A, Yemen—Gunmen on motorcycles shot dead a Yemeni who held a senior security post at the U.S. embassy here, in an assassination that Yemeni officials said bore some hallmarks of an al Qaeda attack.

Qassem Aqlan, a longtime member of the embassy's security staff, was driving along a main road in the capital when he was attacked by several men, who fled unharmed. He died hours later.

The...