Thai forensic officers carry out collected evidence from Phramongkutklao Hospital, a military-owned hospital that is also open to civilians, in Bangkok after a bomb wounded more than 20 people, in Bangkok Monday, May 22, 2017. The deputy commissioner of the Royal Thai Police said investigators found traces of batteries and wires at the scene Monday. (AP Photo/Sakchai Lalit)

Thai forensic officers carry out collected evidence from Phramongkutklao Hospital, a military-owned hospital that is also open to civilians, in Bangkok after a bomb wounded more than 20 people, in Bangkok Monday, May 22, 2017. The deputy commissioner of the Royal Thai Police said investigators found traces of batteries and wires at the scene Monday. (AP Photo/Sakchai Lalit)

BANGKOK (AP) — A human rights watchdog condemned the bombing of a Thai hospital that wounded more than 20 people on the third anniversary of a military coup, saying the blast was an inexcusable crime.

Investigators found remnants of batteries and wires at the scene of Monday’s blast on the ground floor of Phramongkutklao Hospital, police said.

“The bombing of a hospital is an outrageous rights abuse that shows total disregard for human life,” Brad Adams, Asia director at Human Rights Watch, said late Monday. “Bombing hospitals not only risks the lives of patients and medical workers, but disrupts medical care for many more.”

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It was not immediately clear who was behind the explosion or if it was linked to the anniversary of the 2014 military coup that overthrew a democratically elected government.

But the army chief, Gen. Chalermchai Sittisart, said it appeared that the explosion and two earlier blasts in recent weeks used similar explosive materials and were likely part of an attempt to disrupt the government.

“All of this was conducted with the goal of creating disorder to the administrative work of the government and NCPO,” he said, referring to the National Council for Peace and Order, the official name of the ruling junta.

But he cautioned that “we shouldn’t conclude anything yet” about who was behind the attack.

The blast wounded 21 people, one of them severely, said Lt. Gen. Saroj Kiewkajee, a hospital official. Thirteen were discharged soon after the explosion.

Earlier, police said 25 people had been wounded.

Phramongkutklao is a military-run hospital that is also open to civilians. The blast radius from the explosion was 2 to 3 meters (up to 9 feet), police said.

Since the 2014 coup, at least six explosions have occurred in Bangkok.

Last week, a bomb went off in front of the country’s National Theater, wounding two people. Last month, a similar explosion took place in front of an old government lottery office, also wounding two.

Those blasts used similar explosives but did far less physical damage than Monday’s bomb, the army chief said.

“This bomb was meant to cause casualties as it was packed with a large number of nails,” Chalermchai said.

Most of the bombs in Bangkok have caused only minor damage, except for a blast in August 2015 that killed about 20 people near a popular Hindu shrine.