A teacher arrested this week on suspicion of having sex with a student has been under scrutiny for months, a police official said.

In March, the Oro Valley Police Department School Resource Officers Unit began an investigation into a possible inappropriate relationship between Robert Richard Brush, 30, a marketing and media arts teacher at Ironwood Ridge High School, and a female student, Lt. Kara Riley, spokeswoman for the OVPD, said in a news release.

"The investigation was launched immediately once officers became aware of the incident. … However, there was insufficient evidence at that time to make an arrest," she said.

Thursday, the Police Department's Criminal Investigations Unit "received additional information on Brush that they were able to substantiate and gather enough evidence" to make an arrest, Riley said.

Brush was booked into the Pima County jail on suspicion of sexual misconduct with a minor.

Brush, who was hired by the district in August 2012, was placed on administrative leave, said Todd Jaeger, associate superintendent and general counsel for the district. Brush passed a background check when he was hired, the district said in a letter released Friday.

"Certainly nothing in Mr. Brush's background suggested that he was unfit to work as a teacher," the district's letter said.

In his teacher profile on the Ironwood Ridge High website, Brush wrote that he moved to Oro Valley from Washington state with his wife and two children. He has a degree in finance from Portland State University and, at the time of the posting, was pursuing a master's degree in school administration. Before becoming a teacher, Brush worked for six years in commercial real estate investment, development and finance.