TAMPA — A University of Tampa professor tried to pay $140 to have sex with a 14-year-old girl, according to a federal criminal complaint unsealed Monday.

Nathan Madsen, a visiting professor of music at UT, answered an advertisement in the "escorts" section of a website known to host ads for prostitution, the complaint stated.

The ad was placed by a federal agent with Homeland Security Investigations, who posed as a man looking for people to have sex with his girlfriend's daughter.

The complaint details the circumstances that led Madsen, 36, to be charged with sex trafficking of a minor.

In a series of text messages, Madsen negotiated a meeting with the younger of two girls referenced in the ad, according to the complaint. He agreed to pay for 30 minutes with the 14-year-old, the complaint said.

He asked for a picture of the girl. The agent sent a photo. Madsen then asked for a picture that was "nsfw," meaning "not safe for work."

"(D)on't send those kind cause i don't know u," the agent wrote. "Not bout to get in trouble wit that."

"Okay well I'm not going to walk into a trap," Madsen wrote, according to the complaint. "So, pics or voice verify."

Madsen later phoned and spoke with another agent posing as the 14-year-old, the complaint stated. Early Friday, authorities said Madsen drove to an RV park and met with the agent.

"Madsen paid the agent $140 in cash," the complaint stated. "The (agent) asked Madsen not to rough her up or hit her because she had to go to school the next day."

He was arrested Friday. He has since been placed on administrative leave by the university, where he has worked since August 2014, a UT spokesman said.

Madsen's personal website states he has conducted orchestras throughout the United States. He holds a master of fine arts degree from Bard College and, as of 2015, was a Ph.D. candidate at Texas Tech University, according to his online resume.

He was being held Monday in the Pinellas County Jail. Madsen was one of three area men arrested last week by federal agents in similar stings.

James Manning, 24, and Matheus Geronasso, 23, were both jailed Thursday after they separately tried to pay for sex with a 14-year-old, federal officials said. Both also face sex trafficking charges.

Contact Dan Sullivan at dsullivan@tampabay.com or (813) 226-3386. Follow @TimesDan.