Fragments of what appear to be U.S.-made bombs have been found at the scene of one of the most horrific civilian massacres of Saudi Arabia’s 18-month air campaign in Yemen.

Aircraft from the Saudi-led coalition on Saturday bombed a community hall in Sana’a, Yemen’s capital city, where thousands of people had gathered for a funeral for Sheikh Ali al-Rawishan, the father of the rebel-appointed interior minister. The aircraft struck the hall four times, killing more than 140 people and wounding 525. One local health official described the aftermath as “a lake of blood.”

Multiple bomb fragments at the scene appear to confirm the use of American-produced MK-82 guided bombs. One fragment, posted in a picture on the Facebook page of a prominent Yemeni lawyer, says “FOR USE ON MK-82 FIN, GUIDED BOMB.”

ITV News correspondent Neil Connery visited the site shortly after the attack, and found a similar bomb fragment. Connery said he was told by a Yemeni official that it appeared to....