A university instructor told police he killed his girlfriend at the home they shared in Mississippi, where investigators found a note that said “I am so sorry” and gave no hints that he was already headed a few hundred miles north to kill a colleague, police said on Tuesday.

Shannon Lamb called 911 on Monday, telling a dispatcher he had killed 41-year-old Amy Prentiss at the home they shared in Gautier. In the call, Lamb refuses to give his name but says that family contact information can be found on Prentiss’s phone. He says that their dog is still in the house, and “he’s a sweet dog and he’s not going to bother anybody but I’m sure he’s upset”.

It was there that officers found the note, which was written in all capital letters on a white, lined notepad: “I am so sorry I wish I could take it back. I loved Amy and she is the only person who ever loved me.”

Police say Lamb attacked again soon after that Monday morning 911 call, this time shooting Delta State University professor Ethan Schmidt, 39, inside his office. Lamb killed himself hours later as police closed in on him during a manhunt. He had told police that he had no intention of going to jail.

Matt Hoggatt, a spokesman for Gautier police, said during a news conference on Tuesday that Lamb had no criminal record, and there was no indication that he and Prentiss had a history of domestic violence.

Police have not released a motive for either shooting. University president William LaForge said he didn’t know of any conflict between Lamb and Schmidt but “obviously there was something in Mr Lamb’s mind”.

Shannon Lamb is shown in a handout photo provided by the Gautier police department. Photograph: Handout/Reuters

Lamb had earlier asked for a medical leave of absence, saying he had a health issue of some sort, but LaForge gave no further information about it.

The shooting led to an hours-long lockdown at the college during which frightened students and faculty hid in classrooms and closets as authorities scoured the campus looking for Lamb. The campus was eventually cleared by police and authorities later found Lamb when a license plate reader picked up his plate as he crossed a bridge over the Mississippi River from Arkansas back into Mississippi, Cleveland police chief Charles “Buster” Bingham said.

Lamb started working at the university in 2009 and taught geography and education classes. He received a doctorate in education in the spring. He was teaching two online classes this semester, but an in-person class had been canceled, LaForge said.

Lamb’s career prospects at Delta State may have taken a turn because of a university policy change.

After LaForge became president, he hired a new provost, Charles McAdams, who ended a prior university practice whereby an instructor who earned a doctorate could automatically join the tenure track and become an assistant professor. LaForge said that practice violated state policy that requires an open search for new professor positions.

Brandon Beavers, an education major, said he had a class with Lamb last year.

“It was like that class you look forward to,” Beavers said. “It was just cool.”

However, he said Lamb seemed agitated.

“He was really jittery, like there was something wrong with him,” Beavers said. “He was never in a bad mood, but he was real shaky.”

One of Lamb’s longtime friends described him Tuesday as smart, charismatic and funny. Carla Hairston and Lamb both grew up in Greenville, Mississippi.

Hairston said she was 15 and Lamb was 20 when they met through mutual friends. She and her friends were in high school, and he was the cool older guy who tried for several years to teach her to play guitar. He was a good teacher but she was an uncoordinated student, she said.

“He was quite the heartthrob back then. All the girls would melt when he was around,” said Hairston, now 40 and living in the Jackson suburb of Brandon.

“He had the Elvis effect,” Hairston said. “His voice was just like velvet, and people just loved to hear him talk.”

Hairston said even when she wanted to be a rebellious teen and stay out late, Lamb made sure she and her friends went home by curfew. She said he was whip smart and would often quote song lyrics in conversation.

“He made corny and dorky look good,” Hairston said.

Lamb and Prentiss had apparently been dating for some time. Prentiss’ ex-husband said they divorced 15 years ago but remained friends and had a daughter who is now 19.

“She was completely devastated,” he said of his daughter. “She and her mother were absolutely best friends.”

Schmidt, the professor who was killed, directed the first-year seminar program and specialized in Native American and colonial history, said Don Allan Mitchell, an English professor at the school.

Karen Manners Smith, a history professor at Emporia State University in Kansas, where Schmidt studied, called him “a super competent human being”.

“He was president of his fraternity, in student government. He was an absolutely delightful student,” she said.

At the campus of 3,500 students, the police blockades had been removed, people were out cutting the grass and traffic moved normally, although there was not a lot of pedestrian traffic. A vigil was planned for Tuesday night, and classes resume Wednesday.

“We’re trying to get our students to come back,” LaForge said. “The crisis is over. This is a day of healing.”