Last month, the King County Prosecutor formally charged 22-year-old Michael Austin McGrath of Auburn with one count of first-degree arson and four counts of second-degree arson.

On Monday, the Auburn man charged with setting a string of fires in Auburn between last September and early January on Monday pleaded not guilty to the crimes at the Regional Justice Center in Kent.

Police arrested McGrath on Jan. 14 in cooperation with the Federal Bureau of Alcohol, Tobacco, and Firearms (ATF), the FBI, the Valley Regional Fire Authority and other agencies and brought him into custody hours after the reporting of two more north-end fires that had broken out within 15 minutes of each other.

No one was injured in the most recent fires, which damaged a boat and an occupied home.

Detectives later developed sufficient information to find probable cause to arrest a 16-year-old boy, who is McGrath’s cousin. Police said the boy was involved in at least six intentionally set fires, two of which targeted occupied residences. He was booked into the King County Juvenile Detention Center for first-degree arson.

Prosecutors have yet to decide whether to charge the youth as an adult.

Both individuals worked together and separately on several arsons, police said. Auburn Police learned this information while trying to verify details obtained during McGrath’s arrest.

Michael McGrath’s rap sheet includes past charges of second-degree burglary, possession of a stolen vehicle, assault and harassment.