WASHINGTON -- Pope Benedict XVI praised America as a land of opportunity and hope Thursday as he celebrated the first public Mass of his U.S. pilgrimage, but he lamented that the nation's promise fell short for blacks and Indians.

Tens of thousands of worshippers filled Nationals Park on a clear spring day and cheered Benedict as he arrived in a white popemobile, standing in the back and waving. The crowd grew to 46,000, and the demand for tickets doubled the supply, organizers said.

The pope, wearing scarlet vestments, led the service from an altar erected in centerfield of the recently inaugurated baseball stadium. Rows of red-robed church leaders joined him.

In brilliant spring sunshine, the pope walked down from the altar to distribute Holy Communion near the end of Mass.

"Americans have always been a people of hope," he said during his homily. "Your ancestors came to this country with the experience of finding new freedom and opportunity."

DEKALB, Ill. -- Two buildings at Northern Illinois University were evacuated briefly Thursday afternoon because of a bomb threat that was later determined not to be credible, school officials said.

The evacuations did not affect any classrooms, and classes were held as normal at NIU, where a gunman killed five students and himself inside a lecture hall Feb. 14.

Authorities received a bomb threat targeting the health services building about 2 p.m., according to the school's Web site. That building and the adjacent telecommunications building were evacuated but reopened about two hours later.

Campus police declined to comment.

Justin Weaver, managing editor of the NIU student newspaper, the Northern Star, said students weren't certain how to react.

"I don't know if anyone here was ready for this, after the 14th," he said, referring to the Feb. 14 attack at NIU, about 60 miles west of Chicago. "And we just had a vigil last night for Virginia Tech."

ARAPAHO, Okla. -- Authorities have charged a western Oklahoma sheriff with coercing and bribing female inmates so he could use them in a sex-slave operation run out of his jail.

Custer County Sheriff Mike Burgess resigned Wednesday just as state prosecutors filed 35 felony charges against him, including 14 counts of second-degree rape, seven counts of forcible oral sodomy and five counts of bribery by a public official.

Burgess, the top officer in the county of 26,000 since 1994, appeared in court Wednesday was released after posting $50,000 bail.

A message left at Burgess' home Thursday was not immediately returned.

Among other things, Burgess is accused of having sex with a female drug court participant who was in his custody. The crimes are to have occurred between October 2005 and April 2007.

ODESSA, Texas -- A wildfire burned at least four homes Thursday in west Texas and forced the evacuation of about 100 residences, the Texas Forest Service said.

The fire has consumed about 50 acres, the agency said. At least one person was treated for breathing problems, city officials said.

Firefighters were also battling blazes in Colorado and New Mexico.

Snow blanketed the foothills and grasslands on a southern Colorado Army post Thursday where a fast-moving wildfire claimed the life of a firefighting pilot earlier in the week. The fire had forced as many as 800 people to leave their homes.

The blaze, which burned across 15 square miles of Fort Carson, was 20 percent contained. Two other fires in the state this week were fully contained Wednesday, including one that killed two firefighters.

CHICAGO -- Commonly used incontinence drugs may cause memory problems in some older people, a study has found.

"Our message is to be careful when using these medicines," said U.S. Navy neurologist Dr. Jack Tsao, who led the study. "It may be better to use diapers and be able to think clearly than the other way around."

Urinary incontinence sometimes can be resolved with non-drug treatments, he added, so patients should ask about alternatives. Exercises, biofeedback and keeping to a schedule of bathroom breaks work for many.

U.S. sales of prescription drugs to treat urinary problems topped $3 billion in 2007, according to IMS Health, which tracks drug sales. Bladder control trouble affects about one in 10 people age 65 and older, according to the National Institute on Aging, which helped fund the study. Women are more likely to be affected than men. Causes include nerve damage, loss of muscle tone or, in men, enlarged prostate.

ATLANTA -- The current flu season has shaped up to be the worst in four years, partly because the vaccine didn't work well against the viruses that made most people sick, health officials said Thursday.

This season's vaccine was the worst match since 1997-1998, when the vaccine didn't work at all against the circulating virus, according to the Centers for Disease Control and Prevention.

The 2007-2008 season started slowly, peaked in mid-February and seems to be declining, although cases are still being reported, CDC officials said.

Based on adult deaths from flu and pneumonia, this season is the worst since 2003-2004 -- another time when the vaccine did not include the exact flu strain responsible for most illnesses.

Each year, health officials -- making essentially an educated guess -- formulate a vaccine against three viruses they think will be circulating. They guess well most of the time, and the vaccine is often between 70 and 90 percent effective.

But this year, two of the three strains were not good matches and the vaccine was only 44 percent effective, according to a study done in Marshfield, Wis. That seemed to match the experience in other parts of the country.