Background: Seung-Hui Cho (in Korean, properly Cho Seung-Hui; January 18, 1984 - April 16, 2007) was a US resident of South Korean origin, a spree killer and mass murderer who killed 32 people and wounded 17 others armed with two semi-automatic pistols on April 16, 2007, at the Virginia Polytechnic Institute and State University in Blacksburg, Virginia. An additional six people were injured jumping from windows to escape. He was a senior-level undergraduate student at the university. The shooting rampage came to be known as the Virginia Tech shooting.
Context: Immediately after the incident, reports carried speculation by Cho's family members in South Korea that he was autistic. However, no known record exists of Cho ever being diagnosed with autism, nor could an autism diagnosis be verified with Cho's parents. The Virginia Tech Review Panel report dismissed an autism diagnosis and experts later doubted the autism claim.  More than four months after the attack, The Wall Street Journal reported on August 20, 2007, that Cho had been diagnosed with selective mutism. The Virginia Tech Review Panel report, also released in August 2007, placed this diagnosis in the spring of Cho's eighth-grade year, and his parents sought treatment for him through medication and therapy. In high school, Cho was placed in special education under the 'emotional disturbance' classification. He was excused from oral presentations and participation in class conversation and received 50 minutes a month of speech therapy. He continued receiving mental health therapy as well until his junior year, when Cho rejected further therapy.  To address his problems, Cho's parents also took him to church. According to a pastor at the Centreville Korean Presbyterian Church, Cho was a smart student who understood the Bible, but he was concerned about Cho's difficulty in speaking to people. The pastor added that, until he saw the video that Cho sent to NBC News, he never heard him say a complete sentence. The pastor also recalled that he told Cho's mother that he speculated Cho was autistic and he asked her to take him to a hospital, but she declined.  Forbidden by federal law to disclose (without Cho's permission) any record of disability or treatment, Westfield officials disclosed none of Cho's speech and anxiety-related problems to Virginia Tech.
Question: What is selective mutism?

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