PHOENIX -- Undercover officers with the Department of Public Safety said they bought 23 purported live grenades over a two-day investigation.

"Our bomb squad technicians later determined that the explosives were removed and these were just shells, but they still could have been converted into dangerous explosives. That's concerning to us," said DPS spokesperson Harold Sanders.

Sanders said officers bought seven grenade shells on Monday for approximately $400 each, then went back to buy 16 more on Tuesday before arresting three suspects.

At least two of those three suspects are confirmed undocumented immigrants, according to Sanders.

Investigators are still looking for a fourth suspect.

Sanders did not know if these grenade shells were the same ones that investigators with the Phoenix Police Department were tracking in late August.

A police sergeant later wrote a memo detailing how his team was in position to buy up to 60 grenades from a seller, but he believed that his lieutenant stopped the investigation because of an internal turf war.

No arrests were ever made in that initial case and no grenades were recovered.

"We learned about these grenades through our own investigation, so I don't know if they're connected to that case," said Sanders.