TIKRIT, Iraq — The concrete platform at the river’s edge is festooned with flowers and streaked with blood. Along a back wall are photographs taken from a video of the horror that unfolded here last year: a procession of Shiite men, shot in the head one by one by Islamic State fighters and shoved into the waters of the Tigris.

“It’s just because we are Shia,” said Halil Kareem Garim, 61, standing near the river as he recalled the cousin he lost. “We don’t have any problems with Sunnis — we are praying to the same God. It is their mentality. They hate us.”

The riverbank has become a memorial of the massacre and a site of Shiite pilgrimage, already taking a prominent place in Iraq’s ledger of sectarian atrocities. In all, roughly 1,700 Shiite military personnel from the Camp Speicher base are believed to have been methodically gunned down by the Sunni extremists at Saddam Hussein’s old palace complex in Tikrit last June.