Dennis Patrick O’Brien, a former Baltimore Sun reporter who covered the major news stories of the 1990s and later wrote about science, died of a heart attack Feb. 11 at his Washington, D.C., home. The former Ellicott City resident was 69.

“Dennis had great integrity as a journalist,” said David Michael Ettlin, a retired Sun editor. “He was also one of the least boisterous reporters I’ve ever met. He was a modest guy who did a great job.”

Born in Scranton, Pennsylvania, he was the son of attorney Robert E. O’Brien and his wife, Mary. As a student at Bishop Hannon High School, he was captain of the Golden Lancers basketball team when it earned a 1973 state championship.

He earned his bachelor’s degree from American University and immediately went into journalism, initially working at the Manahawkin Beacon and Beach Haven Times, in New Jersey. He later joined the old Baltimore News American where he covered news until its 1986 closure. He then joined the Hartford Courant and later, The Baltimore Sun.

Mr. O’Brien covered the December 1993 sinking of a fishing party boat, the El Toro II, off Point Lookout in the Chesapeake Bay. More than 20 persons were aboard and two died of hypothermia.

“His coverage of the subsequent investigation into the fatal accident (and lawsuit) was thorough and riveting,” said a colleague, Candus Thomson. “He wore a button-up cardigan sweater, one better looking than the last one. He was efficient and laid out his work week well.”

“He was not a pushover. Politicians said he was too hard on them. I would just laugh,” said Ms. Thomson. “He could be equally hard on Sun editors.”

In 1990, Mr. O’Brien began working in The Sun’s Baltimore County bureau.

“He was a fine reporter and an energetic reporter,” said Robert A. Erlandson, a colleague from the Towson office.

While in Baltimore County, he covered Joseph C. Palczynski’s days-long standoff with police in Dundalk. He also wrote of the 2000 murder of Baltimore County Police Sgt. Bruce A. Prothero, who was working an assignment at a Reisterstown Road jewelry store. Mr. O’Brien was present in Philadelphia when key arrests in the case were made.

“Dennis did everything with his trademark calm, deliberate demeanor. He was a real pro, a decent soul, and a good friend,” said a colleague, Jonathan Bor. “You could call on Dennis to do everything. He was hardworking and had his nose to the grindstone. And he was quiet, unassuming and modest.”

While working for The Sun, he earned a law degree from the University of Baltimore and passed the Maryland Bar, although he never practiced.

“He was a terrific court reporter,” said Dan Thanh Dang, a newsroom colleague. “He had so much heart. He cared about people and he cared about getting a story right. His in-depth stories were an amazing read. He wrote fast and was always careful.

“Dennis was generous. Whatever story you were working on, he would help you make it better,” he said.

After leaving The Sun in 2008, he joined the U.S. Agriculture Department in Greenbelt and wrote for its publications.

“He made that job an eye-opening and entertaining beat that drew praise from readers and won the admiration of scientists and researchers,” said Ms. Thomson.

After his retirement, he spent his time swimming, writing a work of detective fiction and exploring Washington and the Adams Morgan neighborhood where he lived. He also enjoyed time with his grandson, Patrick.

Survivors include two daughters, Elizabeth O’Brien of Minneapolis and Kathleen Hernandez of Drexel Hill, Pennsylvania; a former spouse, Sharon O’Brien; two brothers, Kevin O’Brien of Washington and Robert A. O’Brien of Huntington, Long Island, New York; three sisters, Ellen O’Brien, Joan Volz and Maureen McAliney, all of Scranton; and two grandsons.

A Mass of Christian burial was held Friday in Scranton.