(METRO) On the morning of Sept. 11, 2001, 19 terrorists hijacked four commercial airplanes bound for California. The planes departed from airports in Boston; Newark, N.J.; and Washington, D.C.

September 11 would become an infamous date in American and world history, and the events of that day would forever change the world. As the world commemorates the 20th anniversary of 9/11, the following timeline, courtesy of the National September 11 Memorial & Museum in New York City, can help people fully understand how events unfolded on that late-summer morning two decades ago.

—5:45 a.m.: Two of the hijackers pass through security at Portland International Airport in Maine. The men will take a short flight to Boston Logan International Airport, where they will join three other hijackers and board American Airlines Flight 11.

—6 a.m.: Two of the hijacked planes, American Airlines Flight 11 and United Airlines Flight 175, will eventually crash into the North and South Towers of t...