When David Jacobs, a personal trainer and supplement-store owner from Plano, Tex., pleaded guilty to steroid-distribution charges in November and agreed to cooperate with federal authorities, he told a local television station that he would offer evidence linking football players to performance-enhancing drugs.

Five months later, information from Jacobs has led authorities to subpoena at least one active N.F.L. player — a member of the Atlanta Falcons — to testify before a federal grand jury in Texas, said two lawyers with direct knowledge of the matter. The player has not been publicly identified.

The lawyers also said that one of the subjects of the grand jury's investigation was Matt Lehr, an offensive lineman for the New Orleans Saints who played for the Dallas Cowboys from 2001 to 2004 and for the Falcons from 2005 to 2006. The N.F.L. suspended him for four games in 2006 after he tested positive for a banned substance.