The traffic stop that led to the discovery of the devices at about 2 p.m. Wednesday was part of an investigation into a string of burglaries.
Officers identified their first suspect, Dana Sacoman, after a credit card - taken during a burglary - was used at a store.
Dana Sacoman, Michael Chavez, Bernardo Rodriguez, and Jessica Radford were arrested.
They are suspected of multiple nighttime burglaries in the area. Police say the suspects would steal from homes as the residents slept.
Inside the car, police also discovered potentially explosive air bags and a smoke grenade that was rendered safe by the Boulder County Bomb Squad. Longmont police Cmdr. Jeff Satur says the grenades turned out to be replicas that were reported stolen Wednesday evening following news of the evacuations.
The driver of the car, Jessica Radford, was being held on traffic charges while the passenger of the car, Bernardo Rodriguez, was being held on a felony warrant for failure to appear on a burglary charge.
(KUSA-TV © 2012 Multimedia Holdings Corporation with The Associated Press)