From https://pubchem.ncbi.nlm.nih.gov/

Ammonium nitrate by itself does not burn, but in contact with other combustible materials, it increases the fire hazard. It can support and intensify a fire even in the absence of air. Fires involving ammonium nitrate can release toxic nitrogen oxides and ammonia. A fire involving ammonium nitrate in an enclosed space could lead to an explosion. Closed containers may rupture violently when heated.

EPA Office of Solid Waste and Emergency Response; Explosion Hazard from Ammonium Nitrate. EPA 550-F-97-002d, p. 5 (December 1997)
During a fire in a facility where ammonium nitrate is present, the ammonium nitrate can become hot and molten which makes the material very sensitive to shock and detonation, particularly if it becomes contaminated with incompatible material such as combustibles, flammable liquids, acids, chlorates, chlorides, sulfur, metals, charcoal, sawdust, etc. If a molten mass becomes confined (e.g., in drains, pipes or machinery), it can explode.

EPA/OSHA/ATF; Chemical Advisory: Safe Storage, Handling, and Management of Ammonium Nitrate EPA 550-S-13-001, p.6 (August 2013). Available from, as of November 21, 2013: http://www.epa.gov/oem/docs/chem/AN_advisory.pdf
Solid ammonium nitrate readily absorbs moisture, which can lead to caking, self-compression and self confinement. This in turn increases susceptibility to explosion in a fire.

EPA/OSHA/ATF; Chemical Advisory: Safe Storage, Handling, and Management of Ammonium Nitrate EPA 550-S-13-001, p.6 (August 2013). Available from, as of November 21, 2013: http://www.epa.gov/oem/docs/chem/AN_advisory.pdf

On April 18, 1775, British troops march out of Boston on a mission to confiscate the American arsenal at Concord and to capture Patriot leaders Samuel Adams and John Hancock, known to be hiding at Lexington. As the British departed, Boston Patriots Paul Revere and William Dawes set out on horseback from the city to warn Adams and Hancock and rouse the Minutemen. More to come....

On April 18, 1906, at 5:13 a.m., an earthquake estimated at close to 8.0 on the Richter scale strikes San Francisco, killing an estimated 3,000 people. Shock waves are felt from southern Oregon down to Los Angeles, and fires started by the quake in San Francisco will take about a week to extinguish.

On April 18, 1942, 16 American B-25 bombers, launched from the aircraft carrier USS Hornet 650 miles east of Japan and commanded by Lieutenant Colonel James Doolittle, attack targets in and around Tokyo. Several installations are damaged, but more importantly, the attack on the Japanese homeland is a morale boost for American citizens after a string of defeats in the war's opening months.
https://www.youtube.com/watch?time_continue=1&v=Z8PbhaA5bBQ&feature=emb_logo

On April 18, 1983, a suicide bomber drives an explosives-laden car into the U.S. embassy in Beirut, Lebanon, killing himself and 62 people in the embassy, including 17 Americans.


On April 18, 1956, American actress Grace Kelly marries Prince Rainier of Monaco. Kelly, an Oscar winner whose film credits include High Noon and Rear Window, met the Prince while filming the Alfred Hitchcock movie To Catch A Thief! on the French Riviera the previous year. The now-Princess Grace of Monaco retired from acting after the marriage, which was short-lived; Kelly died when her car plunged off a mountain road by the Cote D’Azur in September 1962. She was 52 years old.

At about 5 a.m, April 19, 1775, 700 British troops arrive at Lexington, Massachusetts. Their objective: to capture Patriot leaders there and then advance to nearby Concord to seize a Patriot arsenal. But they arrive in Lexington to find 77 armed minutemen under Captain John Parker waiting for them on the town’s common green. British Major John Pitcairn ordered the outnumbered Patriots to disperse, and after a moment’s hesitation the Americans began to drift off the green. But someone - no one knows who, but American lore suggests it was a Patriot - fires his musket, and others quickly join in. Within moments, eight Americans lay dead or dying and 10 others are wounded. One British soldier is injured, and the regiment moves on Concord. Here they find several hundred armed Patriots waiting. They successfully destroy the arsenal, but take casualties and Pitcairn orders the return to Boston. Its only a 16-mile march, but the British are peppered the entire time by Patriot marksmen firing from concealed positions. By the time the unit arrives in Boston, the casualty count - killed, wounded or missing - is near 300, and the American Revolution is officially begun. (Below: The Battle of Lexington, from the film April Morning)


Just after 9 a.m, April 19, 1995., a massive truck bomb explodes outside the Alfred P. Murrah Federal Building in Oklahoma City, Oklahoma. The blast collapsed the north face of the nine-story building, instantly killing more than 100 people and trapping dozens more in the rubble. Emergency crews raced to Oklahoma City from across the country, and when the rescue effort finally ended two weeks later the death toll stood at 168 people killed, including 19 young children who were in the building’s day-care center at the time of the blast. More to come.....


On April 19, 1993, a 51-day standoff at the Banch Davidian compouned near Waco, TX ends tragically. FBI agents launch a tear-gas assault on the Branch Davidian compound. For reasons that are still unclear, a fire breaks out in the compound, and by the end of the day, the compound was burned to the ground. and some 80 Branch Davidians, including 22 children and the cult's leader, David Koresh, had perished in the inferno. The FBI and the Justice Department maintained that the Branch Davidians likely started the fire themselves in a mass-suicide effort, but later conceded that the tear gas cannisters could have started the fire.

I've heard that too.