DENVER -- Sen. Ben Nighthorse Campbell checked into George Washington Hospital in Washington Thursday morning, suffering from mild chest pains b ut was released later.

His spokeswoman said an the diagnosis indicated he did not suffer a heart attack, but had acid reflux and a spasm of the esophagus, which can mimic a heart attack.

Kate Dando said he was released back to full activities with no restrictions following a checkup.

Campbell, 70, is running this fall for a third term as senator . His health has been scrutinized since he admitted last year that he was undergoing treatment for prostate cancer.

Although he is just in the beginning stages of his re-election campaign, his office has been in the news lately. Recent reports allege that his former chief of staff, Ginnie Kontnik, gave her assistant an inflated salary in 2002 on the condition that he return $2,000 of the money to her. Kontnik resigned her $142,000-a-year position Friday, citing personal reasons.

She said the money was for legitimate Senate expenses and denied the arrangement was a kickback.

The case been referred to the Department of Justice for investigation.