DETROIT (Reuters) - Federal authorities arrested 12 people on Thursday, including 10 current or former Delta Air Lines baggage handlers, in an internal probe of drug smuggling at Detroit Metro Airport, a federal spokesman said.

The investigation continued for more than a year and covered smuggling of cocaine and marijuana, said Khaalid Walls, a spokesman for U.S. Immigration and Customs Enforcement.

Authorities arrested 10 people in the Detroit area, one in California and one in Houston, Walls said.

Prosecutors were in the process of unsealing complaints related to the investigation on Thursday. Representatives from U.S. Immigration and Customs Enforcement and the Attorney's Office in Detroit planned a news conference for later on Thursday.

Delta said in a statement that it had cooperated fully with federal authorities and employees allegedly involved had been suspended without pay. Additional discipline could be taken pending the outcome of the investigation, Delta said.

(Writing by David Bailey; Editing by Jerry Norton)