Bad economic news from China and the bombing at the Boston Marathon drove down the market, contributing to a terrible day for the 50 largest public companies in the greater San Fernando Valley.

All but a handful of the companies saw their shares decline, with Sylmar construction firm Tutor Perini Inc. the biggest loser among larger companies. It fell nearly 9 percent, or $1.63, to close at $16.33.

The biggest dollar loser was Public Storage Inc., the Glendale real estate investment trust that operates storage units. It lost $3.46, or 2.2 percent, to close at $155.05.

Walt Disney Co., the Burbank media giant that is the Valley's largest company by market capitalization, fell $1.67, or 2.8 percent, to close at $58.88. Amgen Inc., the Thousand Oaks biotech and second-largest company, lost $3.09, or 2.8 percent, to close at $108.48.

Among other companies with market caps above $1 billion, Calabasas temporary employment firm On Assignment Inc. lost $1.71, or 7 percent, to close at $22.53. Placement firms generally are seen as barometers of the market given their tendency to reflect general economic conditions.

In the broader market, the Dow Jones industrials lost 1.78 percent and the Nasdaq fell 2.38 percent for the day.

The selling started early after China reported that its economy grew 7.7 percent in the first quarter, less than economists expected and down from a year ago. Later in the day, two bombs exploded at the finish line of Boston Marathon in what is feared to be a terrorist attack.