No injuries were reported in either explosion, a Public Information Officer with the Las Cruces Police Department told ABC News.
The first explosion was reported shortly after 8 a.m. at a mailbox near an entrance to the Calvary Baptist Church, the Las Cruces police reported. Some people were inside the church at the time, but services had not yet begun, police said.
Dennis Llewellyn said he was handing out pamphlets at Calvary Baptist, preparing for the 8:30 a.m. service, when he heard a "boom."
"It was in a mailbox that exploded completely," he said. "The mailbox was completely opened up and twisted and blown apart. It just obliterated everything. If anyone was in front of it, it would have killed them."
Llewellyn, who described himself as a former Marine, said the blast sounded like a Claymore.
Less than 30 minutes later, an explosive device detonated in a trash bin outside the Holy Cross Catholic Church, police said. Church services were already under way at the time.
"That explosion occurred near the front entrance in a -- like a Rubbermaid-style trash can that was outside of the front doors of that church," Dan Trujillo of the Las Cruces Police Department said. "The trash can was right near a front entranceway and had there been somebody within close proximity, they may have well been injured."
Monsignor John Anderson told the Las Cruces Sun-News he heard a loud "pow" sound.
"I knew it had to be more than a gunshot," he said.
Parishioner Ann Marie Sullivan said, according to the Sun-News, "It sounded like something had fallen off and shattered the glass in the back."
Minor damage was reported at both churches, and both have been closed, police said.
The cause is under investigation and there are currently no suspects, police said.
The FBI, ATF, Department of Homeland Security, New Mexico State University Police, state police and local authorities were involved in the investigation.
"We'll look into them to see if they are connected," Trujillo said. "It seems like they might be."