TUCSON, Ariz. -- One illegal immigrants are dead, three are injured and others may be being held captive after gunmen opened fire on a pickup truck near Tucson on Thursday.

Authorities are are now trying to determine who the gunmen were.

The shooting took place along a known smuggling corridor, Pima County sheriff's officials said.

News of the shooting comes as Border Patrol apprehensions continue to drop along the Mexican border in Arizona and elsewhere.

The Border Patrol uses apprehension numbers as a measure of the flow of illegal immigrants across the border. If apprehension numbers go down, the nu7mber of illegal corder crossers is assumed to be going down as well.

The Border Patrol said the new numbers are a sign that changes in policy and increased resources are paying off.

Arrests of illegal immigrants from Texas to California have plunged 27 percent during the first four months of the current fiscal year compared to the same period in fiscal 2006.

So far this year in Arizona, apprehensions have fallen by 9 percent in the Border Patrol's Tucson Sector.

For several years, the Tucson sector was the nation's busiest for illegal immigrant traffic.

In the Yuma Sector, apprehensions fell 62 percent over the same four-month period.