An Indiana sheriff's office is investigating alleged misconduct by the pastor of an evangelical megachurch who was fired after admitting to an "improper relationship" with a teenage girl, the Hammond Post-Tribune reports.

Jack Schaap, 54, was pastor at 15,000-congregation First Baptist Church of Hammond, Ind., for 11 years and broadcast his sermons worldwide.

The church said in a news release that Schaap committed "a sin that has caused him to forfeit his right to be our pastor," the Associated Press reports.

The Lake County Sheriff's Department says it is investigating the alleged misconduct but does not expect to file charges, the Post-Tribune reports.

A church spokesman says the teenager is 17 years old and was at least 16 years old -- the age of consent in Indiana -- at the time of the incident.

The board of deacons announced Schaap's dismissal Tuesday for sinful behavior that involved adultery and physically inappropriate behavior, The Hammond Times reports.

Terry Duff, chairman of the Board of Deacons, said the church is committed to finding out the full story on the allegations and that "no wronging will be covered up."

"We are going to do what is right no matter what the cost or consequences to First Baptist Church," Duff said Wednesday evening.

Terry Duff, chairman of the board, said the church is committed to finding out the full story on the relationship and that "no wronging will be covered up," WBBM-TV, the CBS affiliate in Chicago, reports.

"We are going to do what is right no matter what the cost or consequences to First Baptist Church," Duff told reporters Wednesday evening.

Duff said that Schaap acknowledged the affair when confronted by church leaders and is attempting to reconcile with his wife, who is the daughter of the church's founder.

Correction on 8/3/12: Hyles was not the founder of the church, which dates to 1887, however, he was instrumental in during it into a megachurch during his leadership, from 1959 to 2001.

WBBM reports that interim pastor Eddie Lapina told the congregation Wednesday evening, "We need to watch each other, we need to make sure that nobody … gets the feeling that they're above reproach."

Schaap is also author of several books on marriage and dating, but they are apparently were removed from the website of his publisher, Hyles Publications.

In his 1995 book, titled Marriage: God's Original Intent, Schaap writes:

When it comes to marriage, I want to live by the rules. As a marriage counselors, I'm weary of couples wo come asking me to make exceptions to the rules because they just don't see it the same way God does. .. Nobody has a right to decide if they are an exception to the rule. Decide to be a person who lives by the rules.

A webpage for the purchase of CDs and DVDs of Schaap's sermons is still accessible, however, as is a page on a book Schaap co-authored on teaching in Sunday school.