Officer Mateer was shot and killed as he and his partner, Officer Andres, responded to a robbery at Grand and Olive Street. The two officers were alternating directing traffic at Grand and Olive when a citizen approached Officer Andres and advised him that a robbery was in progress at the Beers Hotel Cigar Company at 3609 Olive. As the officers entered the store they were confronted with the two armed bandits and demanded their surrender. The suspects opened fire with their revolvers, fatally wounding Officer Mateer in the stomach. Both Officer Mateer and his partner were able to return fire but did not strike the suspects. The suspects escaped, running across the street and were never apprehended.

Witnesses described the gunmen as well dressed foreigners. As they escaped one was seen boarding an eastbound streetcar, the other running west on Olive Street. The robbery netted the bandits $180 in cash and a diamond ring worth $100.

Officer Mateer was with the agency for nine years, joining the department in 1916. He was survived by his wife, Catherine, and two children. Interred: Calvary Cemetery.