The Pennsylvania board governing attorney conduct said Tuesday it expects to investigate a Philadelphia lawyer after receiving an unusual referral from the U.S. Supreme Court.

In a rare move, the nation's highest court asked the board on Monday to look into a death penalty appeal the lawyer filed for condemned mass killer Michael Ballard. Mr. Ballard told the court he never authorized the appeal and said in a published interview that he accepts death.

"I'm sure we would open the case," said Paul Killion, chief disciplinary counsel to the board. He said board officials can't recall a similar request from the nation's high court over the past 30 years.

Such referrals are "highly unusual," said Ellen Yaroshefsky, professor at the Cardozo School of Law and director of its ethics center. She said she wasn't familiar with the case but expressed concern that a lawyer could be punished for aggressively protecting a defendant's rights. "We so often criticize defense lawyers for engaging in incompetent behavior by not filing cases," she said.

The Supreme Court declined in late June to consider the appeal, but not before Mr. Ballard—sentenced to die for the 2010 killings of his ex-girlfriend and three others—criticized its filing. Mr. Ballard pleaded guilty to the killings.