D.C. firefighter Donald Derner was relaxing in his Oxon Hill home Wednesday night, watching "St. Elsewhere" on television, when a news bulletin brought a smile of relief to his face.

"Don threw up his arms and said 'That's great,' and I said 'Thank God,' " Derner's wife Ann said yesterday. "We were overjoyed."

But the news about finding Jeremiah Thate also confirmed Derner's worst suspicions about a bizarre scene he had witnessed outside a burning Southeast Washington apartment two weeks ago.

Police discovered Jeremiah, abducted from Prince George's Hospital Center in June, in the same smoke-filled apartment building where Derner and his colleagues had doused a blaze on the chilly night of Oct. 11. It was there that Derner had noticed something odd about two residents standing on the sidewalk.

The two women were black. The baby that one cradled in her arms was white.

Hours before that, the veteran firefighter had lounged in the firehouse where he has worked for 25 years, reading a Washington Post Magazine article about the Thate baby.

As the father of three and grandfather of two, Derner was so moved by the story that he thought about the infant all day. "I agonized over it," Derner said. "The article upset me. If it had been me, I couldn't have handled it as well as the baby's father."

What he saw at the apartment seemed almost unreal.

"It stuck out like a sore thumb," said Derner. "There were two very black women holding a very white baby. It struck me as funny. I had a suspicion that this was the baby in the article."

As he edged closer to them that night, Derner peered into the younger woman's arms to get a better look at the baby.

Dressed in a blue outfit with a little blue hat, the infant was bundled up in about four blankets, he recalled. The older woman suddenly snatched the child out of the younger woman's arms, mumbled something about taking him inside and walked away. But she didn't head into the Howard Road apartment building. She took off down Howard Road toward South Capitol Street.

"That was even more fishy," said Derner. "There was nothing over in that direction except Metro construction."

Later that night, Derner called the phone number listed in the Post article, and was told that a Prince George's County police officer would call him back the next afternoon.

The call never came. So the next day Derner dialed the police again and gave them the address of the women's apartment. An officer told Derner that he would get back to him. Two weeks passed, and the police never called.

"We got the feeling that nothing was going to be done about it," said Ann Derner. "We discussed the baby every day, and thought about calling the missing children's bureau."

Prince George's investigators said a deluge of calls followed publication of the article, and they followed up on Derner's tip after receiving a similar tip eight days later.

"I'm glad they got that baby back," said Derner. "I was just doing what I should do."

Derner has been chasing fires since he was a 9-year-old junior volunteer firefighter in Buffalo except for the four years he spent playing trumpet in the U.S. Naval School of Music and two years in a professional band.

Based at Engine Co. 15 at 14th and V streets SE, Derner serves as an administrative aide to 3rd Battalion Chief Edward S. Garvis. His main responsibilities include driving Garvis to fires, checking out the fire scene and reporting to the fire chief by radio.

A former Boy Scout leader, Derner is described by his wife as a family man who loves to spend time with his grandchildren and always goes out of the way for people, often stopping along the highway to help someone change a tire.

She said he did not even know about the $16,000 reward police offered through Crimesolvers for clues to find the baby.

"He has always been a caring person," Ann Derner said. "That's just the way he is."