U.S. stocks ended at session lows in a disappointing start to the week. The Federal Reserve's June meeting, which kicks off on Tuesday, loomed large, while oil endured its third straight session in decline. Apple's (AAPL - Get Report)  digital assistant Siri has been expanded to use on Macs. The tech giant also offered up Siri to app developers, allowing voice-command tech to be integrated into third-party apps. Marinus Pharmaceuticals (MRNS)  slumped after a late-stage drug trial for epilepsy failed to meet its goals. The treatment was designed for adults who suffered from drug-resistant seizures. Gunmakers including Smith Wesson (SWHC)  and Sturm, Ruger Co. (RGR - Get Report)  closed higher in the wake of the weekend's shooting in Orlando. The attack of a nightclub proved the deadliest shooting in modern U.S. history with nearly 50 fatalities. TheStreet's Keris Lahiff reports from Wall Street.