As the real estate market in the US continued to soar in the early 21st century, banks developed specialized home loan programs for borrowers with lower credit. Lending money to such borrowers, then packaging the loans as mortgage-backed securities to be sold to investors and other financial institutions, became known as "subprime mortgages".

Lehman Brothers, the fourth-largest investment bank in the US, was one of the major factors in the crisis. Lehman focused on developing subprime mortgage securities; between 2003 and 2004, they acquired five mortgage loan institutions. In 2007, Lehman's net income reached a record high of US$4.2 billion in subprime mortgages alone. However, in the same year, the real estate market in the United States began to decline just as a property bubble was emerging. This led to a subprime mortgage crisis in August. Consequently, Lehman Brothers' stock price plummeted, and the value of subprime mortgage securities and bonds fell to the point that they weren't worth the paper they were printed on. This caused investors to lose faith in securities and become unwilling to buy them.

In 2008, the US Department of the Treasury, Bank of America, and the UK's Barclays bank abandoned acquisition attempts, one after the other. Lehman Brothers then declared bankruptcy on September 15th. The incident exacerbated the global financial crisis, causing global stock markets to plummet and 24 countries to suffer greatly.

References: [風傳媒 ("Storm Media Group")(Chinese)] (https://www.storm.mg/article/490613?page=1); [BBC中文網("BBC News Chinese Edition")(Chinese)] (http://news.bbc.co.uk/chinese/trad/hi/newsid_7080000/newsid_7081600/7081616.stm)