Why do they call a murder a "red ball" case?

Not being a native speaker, I wonder why they call a murder a "red ball" case ("redball" or "red-ball") in certain TV-shows and films. Specifically this expression is often heard in TV-shows like The Wire and not least in the film Minority Report (2002). Where does the expression come from?

In the nineteenth century, a common signalling arrangement was a ball hoisted on a mast or pole. These were for daylight communication with ships and later for railroads. Two common ball colors used were red and white. There were no industry-wide meanings for such signals; they depended on the rules of the port or road for definition. Typical uses were to show drawbridge status and to require waiting for a train from the opposite direction (as here).

Rail shipment delays were a problem for the railroads, particularly for perishable produce shipments from the west to eastern markets. In 1892, parts of the Santa Fe Railroad implemented priority treatment for certain designated trains. Whether this used, or just made reference to, a red ball track signal for the fast freight is unclear, but in any case the special trains were called red ball trains and came to have each car carry a placard with routing data and a large red ball (disc) printed on them.(source) By 1902, the Santa Fe had this system running on all its main lines, and by a decade later many railroads were using similar priority programs, often called "red ball" systems.

This railroad usage of red ball for fast transportation became widely known (since the railroads were important in that period). As trucking became established, some companies took on the term in their names: both a Red Ball Transit Company and a Red Ball Motor Freight were formed in the 1920s. Probably the most famous use of the term is the Red Ball Express, which was the truck routing system used in the Normandy campaign of World War II. This symbolic use of red ball for "priority handling" later came to be applied to important criminal cases.

The meaning of the expression Red Ball as a synonym for high priority coming from the railroad industry has been covered by some answers already. As for the use in the criminal investigation area, wikipedia identifies its origin as the book "Homicide: A Year on the Killing Streets".

"Red Ball" is police slang for a high profile case that draws media and political attention. It was popularized in David Simon's book Homicide: A Year on the Killing Streets.

And in the slang glossary of the corresponding article about "Homicide: A Year on the Killing Streets", one can read:

Red ball - A high-profile case that draws media and political attention. Red ball cases are investigated by all detectives on a shift and take precedence over existing active cases. They can and often do make or break a detective's career. They are also known as "shitstorms" and "clusterfucks." Examples during the book include the Latonya Wallace case (Pellegrini's first assignment as primary detective) and the Scott police-involved shooting. Red balls also include major cases that usually fall outside Homicide's jurisdiction, such as non fatal police shootings.

From the alt.tv.homicide FAQ:

As an aside, there has been much conjecture and debate about the origin of the phrase “red-ball.” Following are the two most authoritative (sounding) theories I've yet seen, both contributed by posters on a.t.h.

• FWIW, “Red Ball Express” was the name of a commercial freight firm (which may still exist) that was noted for quick delivery via trucks during the 1930s. During the breakout from Normandy in the summer of 1944 (WWII), “The Red Ball Express” was the nickname given the transportation units that sped supplies from the beachheads and other captured ports to the increasingly distant fighting units.
• I do believe that a red ball was originally a railroading expression; a redball had the priority right of way on the tracks. In other cities these type cases are sometimes called “heaters” (it tends to get very warm) or “apes” (Acute Political Emergencies).