Francisco de Haro and his twin brother Ramon de Haro were the eldest sons and the first set of twins born to Don Francisco de Haro and Emiliana "Miliana" de los Reyes Sanchez. Their mother Miliana was a direct descendent of colonists who had arrived in Alta California with the Anza Expedition of 1776: her grandfather was Jose Antonio Sanchez, aged 29 during the Anza Expedition, and her father was also named Jose Antonio Sanchez, aged 2 during the Anza Expedition.

The de Haro twin brothers were born in Yerba Buena in Alta California, Mexico (later, San Francisco, California, United States), where their father and their maternal uncles served as alcalde (equivalent to mayor).

At the age of 18, the twins acquired rights to the Potrero Ranchero, a large amount of land in present-day San Francisco. Thus, they were wealthy Californios (inhabitants of independent Alta California, of Spanish descent).

The twins were murdered, along with their "uncle" (a cousin on their mother's side) José de los Reyes Berreyesa, while traveling to Sonoma from San Francisco to visit family members and three of Berreyesa's sons, one of whom was the alcalde of Sonoma. Berreyesa's three sons had been made political prisoners in Sonoma during the brief "Bear Flag Revolt" -- when a relatively small group of illegal aliens living in the Central Valley of California set out to overthrow the independent government of Alta California.

The murderer of the three men was a foreigner named John C. Fremont, a U.S. explorer and "Indian killer" who was responsible for a security detail which was accompanying a group of land surveyors as they traveled through Alta California.

Fremont directed three of his employees, one of them known to be the adventurer and hired gun Kit Carson, to kill the unarmed men early one morning, just as they had departed from a rowboat, saddles in hand, after having crossed San Pablo Bay with their elderly uncle. The three men had spent the previous night with relatives at the San Pablo Ranchero (present day El Cerrito in Contra Costa County), on the other side of the bay.

When the elderly Berreyesa asked the murderers why the boys had been killed for apparently no reason, the killers responded by shooting Berreyesa dead. The killers then stripped the clothing and possessions from the bodies of the murder victims, and left the bodies to be buried by the local Native Americans living at the Mission San Rafael Arcangel.

The de Haro twins' ownership of Potrero Hill in San Francisco resulted in the U.S. Supreme Court opinion, De Haro v. United States, 72 U.S. 5 Wall. 599 (1866), issued 20 years after their death.

In addition, evidence of this cold-blooded murder was later presented by a Southern California newspaper, which published an eye-witness account by the respected and well-known surveyor/landowner/politician Jasper O'Farrell, when John C. Fremont ran for election in 1856 as the Republican party candidate for U.S. president, against James Buchanan.

Fremont lost the California vote, and the presidential election went to Buchanan, in part because of what Fremont's supporters called a "military blunder." Even then, people were more than willing to look away when people of color were brutally murdered in cold blood for no apparent reason.

It was only one of many, many incidences in Fremont's long and ambitious career where he had shown utter disregard for human life and for the rule of law.