In February 2001, Joshua Harris Prager of the Wall Street Journal reported that the Giants had positioned coach Herman Franks with a telescope in the Giants' clubhouse during the latter half of the season, including the game itself, and had stolen the pitching signs of the Dodger catcher, Rube Walker, subbing for the injured Roy Campanella in the playoff game. Prager concluded that the spy had signalled pitches to the Giants' batters, including Thomson, thus enabling Thomson to know in advance what pitch Branca was going to throw him. According to Prager's research, Franks was hidden in Giant manager Leo Durocher's office, which was positioned in the Polo Grounds center field and offered a line-of-sight view of the catcher. A buzzer system was installed so that Franks could signal a player in the Giants' bullpen, located on the field of play in deep left field. The player would then signal the batter as to what pitch was coming.
However, acknowledging that sign-stealing was not made a violation of rules by Major League Baseball, and that it had been a part of baseball since the inception of signs as a means of communication between pitcher and catcher, Prager in an interview with CNN on February 3, 2001, left it to readers to determine if the sign-stealing, which Thomson denied, diminished the stature of the event. While the Prager article said that MLB had formally outlawed sign-stealing in the 1960s, his followup book in 2006, The Echoing Green, notes that the major leagues to this writing have not outlawed the practice.
The burden of uncovering sign-stealers is consigned to the opposing team, typically the visiting team. The fact that the visiting teams won the first two games of the playoff series raises the question of how effective the alleged sign-stealing really was. Nonetheless, Prager points out in The Echoing Green that Thompson hit over .100 higher after the sign stealing scheme began in July 1951 and "no doubt" received advanced notice of the two fastballs Branca threw at him that day.