TUCSON — Prosecutors charged Jared L. Loughner, a troubled 22-year-old college dropout, with five federal counts on Sunday, including the attempted assassination of a member of Congress, in connection with a shooting rampage on Saturday morning that left six people dead and 14 wounded.

Evidence seized from Mr. Loughner’s home, about five miles from the shooting, indicated that he had planned to kill Representative Gabrielle Giffords, Democrat of Arizona, according to documents filed in Federal District Court in Phoenix.

Special Agent Tony M. Taylor Jr. of the F.B.I. said in an affidavit that an envelope found in a safe in the home bore these handwritten words: “I planned ahead,” “My assassination” and “Giffords.”

Mr. Loughner, who is believed to have acted alone, is in federal custody and is scheduled to make his first court appearance before a magistrate judge in Phoenix on Monday.

Ms. Giffords was in critical condition after surviving, against the odds, a single gunshot wound to the head at point-blank range. Her doctors were cautiously optimistic that she would survive, and said on Sunday that they had removed nearly half of her skull to prevent damage from the swelling of her brain.

An outpouring of grief was on display all over Tucson, where friends of the many victims joined complete strangers in lighting candles and offering tear-filled prayers. From the back of the temple Ms. Giffords attends, Naomi Present, the distraught daughter of a rabbi, cried out on Sunday morning, “Why, why, why, why?”

Many across America were asking the same thing, and the state found itself on the defensive, with its top lawmakers asserting that Arizona was not a hothouse of ugly rhetoric. President Obama called on Americans to observe a moment of silence at 11 a.m. Monday in honor of the wounded and dead.

Robert S. Mueller III, the director of the F.B.I., traveled to Tucson to oversee the shooting investigation at Mr. Obama’s request. He said an intensive investigation was seeking to determine “why someone would commit such a heinous act and whether anyone else was involved.” Mr. Mueller added that discussions were under way to increase security for all members of Congress.

Capitol security agencies are planning to join the F.B.I. on Wednesday in a security briefing for members of Congress. Already, the United States Marshals Service has increased protection for federal judges in Arizona.

Investigators here focused their attention on Mr. Loughner, whom they accused of methodically planning the shootings, which occurred outside a supermarket. The court documents said Mr. Loughner bought the semiautomatic Glock pistol used in the shooting at Sportsman’s Warehouse, which sells hunting and fishing gear, on Nov. 30 in Tucson.