Federal officials say the number of "drop houses" discovered harboring illegal immigrants in the Phoenix area has decreased significantly in the last four years.

They say the decrease is another indication that overall human smuggling activity in Arizona is declining.

The Arizona Republic reports just 490 illegal immigrants were discovered in 37 Phoenix area drop houses in the federal fiscal year that ended Sept. 30.

That compares with 3,221 illegal immigrants found in 186 drop houses in the 2008 federal fiscal year.

Officials say the decrease in smuggling activity in Arizona is tied to an overall drop in illegal immigration because of the weak U.S. economy, tighter border security, stepped-up immigration enforcement and tougher sentences imposed on smugglers.

Smugglers use drop houses to put up illegal immigrants until they can be moved to their ultimate destinations. Conditions in the houses often are harsh.