BOSTON (AP) -- A Boston man charged with fatally shooting his father has been ordered held without bail.

A not guilty plea to a charge of murder was entered on behalf of 33-year-old Mark Tomas Regan at his arraignment Monday.

Authorities say the younger man shot and killed 66-year-old Mark Thomas Regan sometime last week. Friends of the older man went to the family’s West Roxbury home last Wednesday after he failed to show up for work. Police who entered the home during a well-being check Friday found the body of the elder Regan.

They also found the younger man in the attic. A gun was also found in the attic.

The younger Regan’s lawyer says his client denies the charges.

Authorities did not disclose a possible motive.