Contacts in US Customs and Border protection can be found HERE. They will be able to give you this information. The medical examiners in individual sectors are the ones who record cause of death. I've also attached a graph from the US Census Bureau on estimates of illegal immigrants 1990-2000 (see attached).

The San Diego Independent Media Center also has some background.

While the figures are slightly old now, Dr. Wayne Cornelius, professor of political science and U.S.-Mexico relations at the University of California at San Diego, the director of the Center for Comparative Immigration Studies, as well as the director of the Center for U.S.-Mexican Studies, provided a quantitative analysis of Operation Gatekeeper has published a report entitled, "Death at the Border: The Efficacy and 'Unintended' Consequences of U.S. Immigration Control Policy 1993-2000" that includes statistics.

There is also this site which seems to extrapulate from the previous study's findings and lists deaths by sector.

Read Record number died crossing border in '05 from the San Francisco Chronicle October 5, 2005.

Also these articles are full text in EBSCOhost database, which you may be able to get from your local public or academic library:

• Dying to get in. (cover story). Ufford-Chase, Rick. Christian Century, 8/10/2004, Vol. 121 Issue 16, p20-23, 3p.
• Death on the Border. By: Cooper, Marc. Nation, 10/18/2004, Vol. 279 Issue 12, p8-26, 2p
• Immigration reforms would shrink death toll at border. USA Today, 08/25/2005
• Dangerous desert, breached border; Mexico's northern border. (Insecurity and migration on the Mexican-American border). The Economist (US) Jan 8, 2005 v374 i8408 p38.

also look into the book by Ken Ellingwood entitled, HARD LINE: Life and Death on the U.S. Mexico Border.