The grenades apparently were used in a fight Thursday between members of the Zetas drug cartel and the Gulf cartel, which is trying to reclaim the territory, The Dallas Morning News reported.
Fighting between the two groups, one-time allies, has left a string of burned-out homes and bodies. Sources told the Morning News seven people have been killed in recent days, including three Americans.
Nuevo Laredo, an entry into Texas via Interstate 35, is known as the Zetas' headquarters.
The Morning News said the Gulf cartel is believed to be backed by the Sinaloa crime organization as it tries to eliminate the Zetas and control the profitable drug route into the United States.
"Things will only heat up," one person, speaking anonymously, told the newspaper. "This thing goes in waves, and we're about to see a huge wave."