Three additional charges were lodged against the son of a sheriff's deputy accused of burning down three predominately black churches in southern Louisiana in the past few weeks.

A Louisiana judge added three federal hate crime charges against Holden Matthews, 21, and denied him bond on Monday, multiple media accounts reported, including CBS News.

Matthews, a white resident of St. Landry Parish, the county where the fires occurred, was previously charged with three counts of arson on religious buildings, officials said.

Matthews entered a plea of not guilty on Monday and remains in custody at the St. Landry Parish jail, media reports say. A judge set a September trial date.

Matthews, the son of Deputy Roy Matthews of the St. Landry Parish Sheriff's Office, was taken into custody last Wednesday.

The three churches destroyed by the fires have mostly black congregations, raising authorities' suspicion that the fires may be racially motivated hate crimes.

Noting the history of burnings of black churches in the South, Louisiana Governor John Bel Edwards said the episode was "especially painful because it reminds us of a very dark past of intimidation and fear".

The fires set between March 26 and April 4 destroyed St. Mary Baptist Church in Port Barre, and Greater Union Baptist Church and Mount Pleasant Baptist Church in Opelousas.

St. Landry Parish Sheriff Bobby Guidroz said he believed his deputy was unaware of Holden Matthews' involvement in the fires.