Rapper Nelly was detained last night in Texas while police  officers searched his tour bus -- and found HEROIN in addition to a loaded gun  ... and MASSIVE amounts of weed ... law enforcement sources tell  TMZ.

According to law enforcement sources, Nelly's bus was stopped at a  border patrol checkpoint in Sierra Blanca -- and a police canine detected the  presence of drugs.

We're told cops pulled the bus over -- and after  performing a search, they found 36 small baggies of heroin (totaling .64 ounces)  and over TEN POUNDS of marijuana inside a large green duffel bag (an amount  roughly the size of a large bed pillow).

Cops also say they found a  loaded .45 caliber pistol.

There were seven people riding on Nelly's bus  at the time -- including the rapper -- and they were all detained during the  search.

According to law enforcement sources, one of the passengers -- a  guy named Brian Keith Jones -- admitted the drugs and the gun were his. He was  subsequently placed under arrest.

Nelly and the remaining five were let  go. Calls to Nelly's reps were not returned

Read more: http://www.tmz.com#ixzz291ZsKj1E