AUSTIN (KXAN) — Federal authorities busted a mail-theft ring in Austin and recovered a cache of weapons and stolen checks linked to the robberies of at least two US Postal Service mail carriers, and the attempted robbery of a third less than a week ago, according to newly filed federal court records obtained by KXAN.

“Protecting USPS employees is part of the core mission” of the US Postal Inspection Service, US Postal Inspector and public information officer Silvia Torres said in a statement.

“When a robbery does occur, it receives the highest level of response and attention,” Torres continued. “Postal Inspectors worked tirelessly with our law enforcement partners, including the Austin Police Department (APD), and used every investigative tool at our disposal to identify and arrest the individuals responsible for these criminal acts.”

Federal investigators accuse Camren Hamilton, Christian Mosquera, Cody Smith, Jonathan Rangel and other unnamed “conspirators” of being part of a criminal mail-theft ring.

All four face federal charges of mail theft, robbery and illegal possession of USPS master keys, called “arrow” or “universal.”

The group is accused of stealing the master keys from a letter carrier in Round Rock in September and again in Leander on Dec. 4.

In that case, the mail carrier reported two men demanded he “give up the key” and “ripped” it off him.

The USPIS previously sent out a flyer offering a $150,000 reward for information leading to the suspects’ arrest.

Surveillance videos, rental car records and license plate camera data led federal investigators to Mosquera and Smith, who are also linked to an attempted “arrow key” robbery in Round Rock as recently as Dec. 9, according to the new criminal complaint signed Dec. 14. In the most recent case from last week, the mail carrier screamed for help, and the two suspects ran away when one or more witnesses came to her aid.

On Dec. 13, federal investigators raided three Austin apartments linked to the suspects. At Mosquera’s apartment located at a complex on Spectrum Drive in Austin, USPIS investigators found “at least” 11 semi-automatic rifles, two semi-automatic pistols and one ore more stolen checks. In a separate apartment at the same complex, rented by Smith and Hamilton, a “significant number” of stolen personal and business checks mailed from Leander were found.

Inside Smith’s bedroom, investigators found more stolen mail along with the arrow key from the Dec. 4 robbery of a USPS letter carrier in Leander, according to the federal criminal complaint.

At a separate apartment connected to Mosquera on Crossing Place in Austin, investigators found a handgun, loaded rifle magazines “consistent with the weapons seized” from his other apartment and mail that had been shredded.

Master keys are “greatly coveted within criminal organizations” because they allow thieves to easily open up and empty out blue collection boxes and cluster mailboxes in order to steal checks, money orders, credit and debit cards, tax refunds and sensitive personal information from hundreds, if not thousands, of people. Many people don’t even realize their checks are missing or stolen until they review their bank records, according to the complaint.

KXAN recently obtained surveillance videos from one southwest Austin neighborhood showing different masked thieves using stolen arrow keys to empty cluster mailboxes at least four times since last year.

Residents said the concern for theft is heightened around the holidays. The busiest time of the year for the USPS is the two weeks before Christmas.

This week, federal investigators arrested Mosquera and Smith.

Hamilton and Rangel were arrested on Sept. 30 “within minutes” of stealing an arrow key in Round Rock, according to the complaint.

A handgun retrieved from a car driven by Hamilton was purchased by Mosquera in a private sale using the fake name “Chris Santana.” Investigators also found checks with the payee name changed to Smith’s name. Smith told investigators he and Hamilton were from St. Louis, Missouri and recently relocated to Austin. He “denied any knowledge” of the robberies, mail theft or check fraud.

Both were indicted in October. As of Dec. 14, Rangel was still behind bars while Hamilton was released on bond and allowed to reside with family in St. Louis during detention proceedings.

A September Office of Inspector General Report found more than 400 mail carriers were robbed nationwide last year. The number of USPS employees “assaulted and murdered to obtain” the arrow keys “has increased dramatically” over the last five years, according to the criminal complaint.

The USPIS thanked APD’s violent crime, robbery and SWAT teams for their “invaluable assistance” on this case, adding there was no further information to release about the arrests at this time.

In May, the USPS announced a plan to replace 49,000 “antiquated” arrow locks with new electronic ones and install 12,000 high-security blue collection boxes. The OIG report, however, criticized the USPS for its lack of “deployment timelines with actionable milestones to fully implement planned initiatives to address mail theft” and said it “lacks accountability” for their arrow keys.