ALEXANDRIA, Egypt — When the bomb went off at St. Mark’s Cathedral one week ago, William Frances had one thought: “Oh, my God, it’s happening again.”

Six years earlier, Mr. Frances lost his mother, his sister and a cousin in a bombing at another Alexandria church that left him devastated. Now he prayed he hadn’t lost anyone else.

“I had enough,” he recalled. “I said: ‘Please, God, no more. Please.’”

The coordinated suicide attacks on St. Mark’s, Egypt’s historic seat of Christianity, and at another church, in the city of Tanta, took 45 lives and dealt a heavy blow to the country’s embattled Coptic Orthodox minority. The Islamic State group claimed responsibility for both attacks.

The bombing in Alexandria, a bustling seaport of crumbling elegance, also dredged up painful memories of 2011 church attack that, despite years of investigation, remains unsolved. The trail is stone cold: Not only have the Egyptian police failed to arrest those responsible for the bloodshed, they can’t even say which group carried it out.