Weeping, grieving families gathered outside a Thai kindergarten on Friday where a former police officer murdered nearly two dozen children in one of the kingdom’s worst mass killings.

Thai King Maha Vajiralongkorn and Prime Minister Prayut Chan-O-Cha will later visit survivors of the attack that killed at least 37 people, including the attacker’s wife and child.

Coffins containing the bodies of the victims – including 23 children – arrived overnight at a morgue in Udon Thani, the closest town to the sleepy rural district torn apart by Thursday’s three-hour shooting spree.

Armed with a 9mm pistol and a knife, the fired police sergeant Panya Khamrab opened fire on the child care center in the northeastern province of Nong Bua Lam Phu at around 12:30 p.m. (0530 GMT).

After the attack, Panya, 34, fled the scene in a pickup truck to drive home and murder his wife and child before taking his own life, police said, ending the rampage around 3pm.

Scores of people gathered outside the kindergarten — a low-rise building in a municipal administration complex with a lawn in front of it — on Friday to mourn and pay their respects.

Some cried inconsolably and wiped their eyes as their loved ones hugged them.

Flags on government buildings flew at half-mast on Friday in a gesture of mourning one of the deadliest days in recent Thai history.

Nanthicha Punchum, acting director of the kindergarten, described harrowing scenes when the attacker broke into the building in rural Na Klang district.

“Some employees were having lunch in front of the kindergarten and the attacker parked his car and shot four of them,” she told AFP.

“The shooter smashed his leg in the door and then came in and started slashing the kids’ heads open with a knife.”

Local media reported that among the dead was a school teacher who was eight months pregnant and that one child survived because she was sleeping under a blanket when the attacker struck.

National police chief Damrongsak Kittiprapat told reporters that Panya, a former police sergeant, was suspended in January and fired in June for drug use.

He said the attacker – who used a legally purchased handgun – had been tried on a drug charge, adding that Panya was in a manic state but it was not known if it was drug-related.

Witness Paweena Purichan, 31, said the attacker was a known drug addict in the area.

She told AFP she encountered Panya driving erratically as he fled the scene.

“The attacker rammed a motorcycle into two injured people. I sped off to get away from him,” she said.

“There was blood everywhere.”

Video footage Paweena posted online showed a woman lying injured in a roadside bush after apparently being pushed off her motorcycle by Panya.

Thai Prime Minister Prayut has ordered a swift investigation into the attack, which came less than three years after a 17-hour rampage by a soldier and others wounded 29 people before he was shot dead by commandos.

And less than a month ago, an army officer shot dead two colleagues at a military training ground in the capital, Bangkok.

But while Thailand has a high rate of gun ownership, mass shootings like Thursday’s are rare.