CUMBERLAND, Ind. -- Police staked out a park Monday night, hoping that those responsible for setting off a bomb might return to the scene.

Someone detonated a bomb at Lions Park just after 9 p.m. Sunday, 6News' Ericka Flye reported. Police exploded another bomb that was found in a duffel bag near the first one.

A bomb squad and dogs swept the park and didn't find any more devices. Police said it is safe for people to use the park, but nearby residents jolted by the explosion are still shaken.

The first explosion left a large hole and was heard by residents who live miles from the park.

"My mom thought it was a gunshot, but … it was a lot louder. It was a thick boom," said resident Jerri Coverstone. "It scared us to death."

Residents said they heard and felt the explosion.

"I just opened the back door to check on my dog and all the sudden 'boom,'" said Roxy Cole, who lives nearby. "I saw my dog actually come up … off the ground."

Calls poured in to police. When officers and fire crews arrived, it was difficult to see because it was dark.

Police said they think those who set off the bomb didn't intend to leave a duffel bag with another one just a few feet away.

"The robot was brought in to check out the secondary device," said Cumberland police Officer Bradley Morris. "It turned out that it was another device in a book bag."

Police said the bombs were not near the playground.

"If it's kids that did it, they need to learn to play baseball. We got tennis courts. We got basketball courts. We got a great park here," Coverstone said. "Why are they doing stupid stuff like this?"

Witnesses said they saw two males and two females in two vehicles leave shortly after the blast in a black pickup truck and gold sedan.

Police originally called the devices pipe bombs but said late Monday that they were "improvised explosive devices" and are still processing scattered materials to determine how they were made.