Vikings spokesman Bob Hagan issued a two-sentence statement on Thursday that said the Vikings "are not commenting on the issue. These types of matters are handled by the league."
The Packers filed a tampering charge with the league, contending Favre, who has asked the Packers for his release, has been talking with Vikings offensive coordinator Darrell Bevell. The two became friends when Bevell was an assistant coach with the Packers.
A source in Green Bay told ESPN.com's John Clayton the Packers believe they have a strong case against the Vikings. They believe the Vikings were willing to talk to Favre in order to cause chaos within the Packers family during the summer.
Another source suggested Favre might have had a conversation with Vikings coach Brad Childress.
NFL spokesman Greg Aiello said Wednesday the league had no comment. The Vikings open the season in Green Bay.
Fellow NFL MVP LaDainian Tomlinson of the San Diego Chargers told Bloomberg Television he believes Favre was forced to retire before he was ready to.
"Brett Favre should have the opportunity to control his own destiny,'' the running back and 2006 MVP said Thursday. "If he wants to come back and play football, then let him come back. I think that he was forced to retire prematurely. He finally realized that's not what he wants to do."
"I think they are making a mistake," Tomlinson said of the Packers. "If they don't want [Favre] back, that's one thing. Release him, let him go to another team, but don't tell him he can come back but he's going to be a backup. That's not fair to Brett Favre and everything he's done for the organization."
On June 20, Favre, who retired on March 3, called Packers coach Mike McCarthy and told him he had the "itch" to return to the NFL. On Saturday, Packers general manager Ted Thompson said Favre could return to active status on the Packers if he comes out of retirement, but his role would not be determined. During Favre's retirement, Aaron Rodgers has been deemed Favre's successor.
The Packers said they were ready to welcome Favre back to the team later in March when Favre expressed reservations about his decision to retire, only to be assured by the quarterback that he was finished.
Favre has said that he felt pressured by the Packers to retire and now has been told that if he returns to Green Bay, he wouldn't necessarily get the starting job back.
If the Packers can prove the Vikings tampered with Favre, they could be subject to fines or a possible loss of a draft choice.
Childress called the situation a "soap opera," and said earlier this week that it's been interesting to watch, but he remained committed to going forward with Tarvaris Jackson as the Vikings' starting quarterback.
Bevell was not available to comment on Thursday and neither were any other Vikings officials, Hagan said.
Meanwhile, in Milwaukee, the Favre-Packers feud has begun to weigh on former players in addition to the fan base. Hall of Fame defensive end Willie Davis remains respectful of Favre's iconic status but believes the team has a need to move on.
"It is a bit of sadness," Davis said this week in a telephone interview with The Associated Press. "And it's a bit of sadness because I know how much of a burden this places on everybody."
Davis, an emeritus member of the Packers' board of directors, played for Green Bay from 1960 to '69 and was inducted into the Pro Football Hall of Fame in 1981. From his own experience, he knows how hard it can be for a player to know when to retire.
But he hasn't quite seen anything quite like this.
"It's not only a surprise to me," Davis said. "I cannot believe the magnitude of this thing."
John Clayton is a senior NFL writer for ESPN.com. Information from The Associated Press was used in this report.