A New York district attorney has opened an investigation into the death of the brother of Samuel and Thomas McNell, whose allegations led to the resignation of CIA spymaster Max Hugel.

An aide to Queens District Attorney John Santucci said last night that the New York City medical examiner, Elliott M. Gross, wrote Santucci a letter yesterday suggesting that the body of Dennis McNell, of Queens, be exhumed so an autopsy could be performed.

Dennis McNell, 41, marketing vice president of Triad Energy Corp. his brothers' company, died June 1 of an apparent ruptured spleen after returning home from jogging, complaining of stomach pains and being rushed to a hospital.

Samuel and Thomas McNell disappeared shortly after The Washington Post ran an article detailing their charges that Hugel was involved in stock manipulation.The FBI and the Securities and Exchange Commission is investigating the disappearances.

The Santucci aide said Gross noted that ruptured spleen is often caused by trauma resulting from violence. No autopsy was performed after the death. Santucci is investigating to see if there is some probable cause of foul play before exhuming the body, the aid said.

Stanley Kielmar, a Chicago oil man and a Triad director, said in a telephone interview last night that he heard at an emergency board meeting last week that Dennis McNell had been abducted and beaten twice this year.