Bereaved families held a solemn service Wednesday to mark 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 gathered at the bombing site for a ceremony that included the tolling of a bell and the scattering of flower petals in a memorial garden.

People leave floral tributes at the Omagh Bomb Memorial for the victims of the Omagh bombing in the market town of Omagh, Northern Ireland, Wednesday Aug. 15, 2018, ahead of the ceremony to mark the 20th anniversary of the Omagh bombing on Aug. 15, 1998. Bereaved families are marking the 20th anniversary of the deadliest attack in Northern Ireland's four decades of violence, which left 29 people dead, including a woman pregnant with twins. (Niall CarsonPA via AP)

People leave floral tributes at the Omagh Bomb Memorial for the victims of the Omagh bombing in the market town of Omagh, Northern Ireland, Wednesday Aug. 15, 2018, ahead of the ceremony to mark the 20th anniversary of the Omagh bombing on Aug. 15, 1998. Bereaved families are marking the 20th anniversary of the deadliest attack in Northern Ireland's four decades of violence, which left 29 people dead, including a woman pregnant with twins. (Niall CarsonPA via AP)

Nuala O'Loan, who investigated the bombing when she headed the oversight body for Northern Ireland's police force, said the slaughter could have been prevented if intelligence services had coordinated better and shared information.

In her 2001 report, O'Loan said she didn't know whether it could have been stopped. But on Wednesday she said: "My view now is that it could have been prevented."

Police Service of Northern Ireland chief George Hamilton said O'Loan's comment was "inaccurate, unfair and unreasonable.

People gather on Market Street, Omagh, Northern Ireland, Wednesday Aug. 15, 2018, ahead of the ceremony to mark the 20th anniversary of the Omagh bombing on Aug. 15, 1998, with the glass Omagh Bomb Memorial, centre left. Bereaved families are marking the 20th anniversary of the deadliest attack in Northern Ireland's four decades of violence, which left 29 people dead, including a woman pregnant with twins. (Niall CarsonPA via AP)

People gather on Market Street, Omagh, Northern Ireland, Wednesday Aug. 15, 2018, ahead of the ceremony to mark the 20th anniversary of the Omagh bombing on Aug. 15, 1998, with the glass Omagh Bomb Memorial, centre left. Bereaved families are marking the 20th anniversary of the deadliest attack in Northern Ireland's four decades of violence, which left 29 people dead, including a woman pregnant with twins. (Niall CarsonPA via AP)

"Police were not in a position to prevent the Omagh bombing," he said.