After shooting and killing his girlfriend in Mississippi on Monday morning – and before he shot and killed his colleague later that day – Shannon Lamb wrote a note to say that he was “sorry” for the first murder and wished he “could take it back,” authorities revealed Tuesday.

Police in Gautier, Miss. said Lamb, a geography and social science education instructor at Delta State University, called 911 around 10 a.m. Monday and said he had shot and killed someone.

Lamb told the dispatcher that police needed to send officers over to his home. He said "I shot my wife last night" and also mentioned that there is a "sweet dog" in the house that is "not going to bother anyone" but is likely upset. Police asked the caller his name, but he didn't give it. Lamb did not provide an explanation for the shooting.

When police got to the scene they found the body of Amy Prentiss, 41, and a handwritten note from Lamb, 45, that said: "I am so sorry I wish I could take it back. I loved Amy and she is the only person who ever loved me."

After writing the note, Lamb drove 300 miles to Delta State where he killed Ethan Schmidt, a history professor, before eventually taking his own life late Monday as cops closed in.

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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.”

Despite reports Monday that Lamb allegedly believed Prentiss was also romantically involved with Schmidt, authorities said Tuesday that there was “no information, no evidence” of a “love triangle.”

Prentiss was described as a "good person" by Shawn O'Steen, who said he had been married to her for about seven years. The two divorced 15 years ago, but remained friends. O'Steen said he and Prentiss had a daughter, who is 19, and she was "devastated." He said Prentiss and her daughter were "absolutely best friends."

Prentiss worked an online job from her home, according to Linsday Knowles, identified as a close friend of Prentiss by the Sun Herald. Knowles said Prentiss, who had a hound dog named Lightning, had been dating Lamb for about three years.

"She always wanted to help other people," Knowles said.

Police said Lamb had phoned relatives at some point Monday to tell them “he’s not going to jail.” 

When cops caught up with him on Highway 1 near Greenville later that night, Lamb pulled his car over, bailed out on foot, ran into the woods and shot and killed himself.

Lamb received a doctorate in education from Delta State in the spring of 2015, according to his resume posted on the university's website. He started working there in 2009 and taught geography and education classes, and volunteered with Habitat for Humanity, according to the resume.

LaForge said Lamb was teaching two online classes this semester.

Reports of Schmidt's shooting sent the campus into a lockdown Monday morning. Law enforcement personnel poured over the grounds in search of the gunman.

The 3,500-student university is in Mississippi's flat, agricultural region near the Arkansas state line. Cleveland is a city of about 12,000 people located 38 miles northeast of Greenville.

The Associated Press contributed to this report.