A small “ping pong” bomb hidden in a plastic bag exploded in Bangkok on Friday, wounding a street cleaner, police said, after a spate of small bombs in the Thai capital raised speculation about opposition to military rule.

The blast came a day after authorities said they had arrested a suspect in connection with last month’s explosion at a military-run hospital in Bangkok that wounded more than 20 people.

“It was in a plastic bag hidden in a bush and city cleaning staff went to pick it out and opened it, that was when it exploded,” Kajornpong Jitpakpoom, the police commander in the city’s Thonglor district, told Reuters.

“It doesn’t appear to be a deliberate attack,” he added. “It could be related to students, who sometimes use small bombs like this and hide them.”

The devices are known as “ping pong” bombs as they are about the size of a table tennis ball.

Police bomb squad officers were at the scene, the team’s chief said, adding it was too early to provide details.

No group has claimed responsibility for several small blasts in Bangkok in recent months, but they have fueled speculation that opponents of military rule are making a show of defiance.

The military seized power from an elected government in May 2014, saying it had to step in to end months of at times bloody anti-government protests.

The military has promised to hold an election next year, but critics say it is intent on using a new constitution to maintain control over civilian governments. Continue reading.