Many GardaWorld guards are former Marines or police officers.

“I really don’t understand the rationale for approaching a guy that I know is armed, I know is trained and I know will defend himself and shoot back, if not shoot first,” said Mr. Hatchett, one of two company executives who have been speaking to the news media. “Even if you get the bag, typically it’s a short-lived celebration.”

GardaWorld said it had been particularly outspoken about the Houston problem because of the frequency and violence of the robberies. Armored-truck companies typically refrain from commenting publicly after a robbery, leaving the police to take the lead. Other companies that were robbed in Houston have not granted interviews.

In addition to issuing the public threat to would-be robbers, GardaWorld has increased its security, surveillance and tactical-weapons training; added a third person to the usual two-person crews in some trucks; and put shotgun-wielding guards in so-called chase vehicles to follow the trucks on their routes or monitor their stops. The company has also started a public awareness campaign intended to tear down the stereotype of lackluster rent-a-cops and to reassure its clients and the public.

Jim McGuffey, who spent nearly 30 years in the industry and now runs a security consulting business in South Carolina, was skeptical of the effectiveness of GardaWorld’s campaign. “It’s unusual that a carrier would have a conversation with the press following a robbery,” Mr. McGuffey said. “If that is their style, to try to talk to the bad guys if you will, that’s not going to be as effective as working in collaboration with law enforcement in a proactive fashion.”

GardaWorld said it had been working closely with local and federal authorities, and had gotten a positive response from many in the industry to its campaign, which it said had focused attention on the robberies and helped put a spotlight on a business that is rarely in one.

The authorities have arrested three men so far, charging them with aggravated robbery after the shooting near the drive-through A.T.M. in October. But investigators said they were pursuing other leads and other suspects.

They said they believed that at least six of the robberies were carried out by a loosely organized group, adding that there was nothing particular about the region’s armored-transport companies or its criminal element that would explain the increase in robberies.