Mexican drug cartels, whose trafficking efforts into the U.S. have grown bolder amid the border crisis, are increasingly utilizing a frightening weapon in their arsenal: the hand grenade. Whether packed with explosives, hollowed out and filled with steel to give them authentic heft or even used to deliver handwritten death threats, grenades are turning up in greater frequency in encounters with suspected cartel members, according to law enforcement authorities and border experts. U.S. Immigration and Customs Enforcement officials confirmed to FoxNews.com that they’ve seen a “trend increase” involving the devices along the entire southwest border, although details were not disclosed...