LONDON (AP) — Bereaved families are marking the 20th anniversary of the Omagh bombing, the deadliest attack in Northern Ireland's four decades of violence.

On Aug. 15, 1998, a car bomb ripped through crowds of shoppers, workers and tourists in the market town, killing 29 people including a woman pregnant with twins. Police blame an Irish Republican Army splinter group that was opposed to Northern Ireland's peace process, but no one has been convicted of the bombing.

Relatives are gathering Wednesday at the bombing site for a ceremony that will include the tolling of a bell and the scattering of flower petals in a memorial garden.

Nuala O'Loan, who investigated the bombing when she was Northern Ireland's police watchdog, says she thinks it could have been prevented if intelligence services had coordinated better.