(CNN) -- A veteran police officer in Alaska was behind bars Monday after authorities accused him of passport fraud, using a fake identity and being an illegal immigrant.
Federal prosecutors allege that Rafael Mora-Lopez, a Mexican citizen, was working as a police officer under the false name of Rafael Espinoza.
Assistant U.S. Attorney Thomas C. Bradley said authorities discovered the assumed identity after he applied to renew his U.S. passport in January.
Mora-Lopez pleaded not guilty to a charge of passport fraud in federal court Friday. The Department of State, the Department of Homeland Security and Anchorage Police are still investigating the case, the U.S. Attorney's Office said.
Investigators found "documents confirming Mora-Lopez's true identity" during a search of his home last week, a statement from the U.S. Attorney's Office said.
Passport fraud carries a maximum 10-year sentence, the U.S. Attorney's Office said. Authorities said Mora-Lopez was in the country illegally and could face additional charges, the KTVA television station reported.
After nearly six years on the force, the officer known as Rafael Espinoza had a reputation "as nothing but a sterling employee" among his colleagues at the Anchorage Police Department, Chief Mark Mew said, according to KTVA.
"His problem was, he lied his way into the job," Mew said.
At Friday's arraignment hearing, his attorney said Mora-Lopez was submitting a resignation letter, Bradley said.
Local television stations reported that Mora-Lopez uttered only one sentence as he left the courtroom: "I was pretty proud to serve the community."
Bradley said Mora-Lopez remained in jail Monday.
Mora-Lopez's attorney, Allen Dayan, could not immediately be reached for comment.
His trial is scheduled for June 27.
CNN's Catherine E. Shoichet contributed to this report.