Commander Isiah Nelson, a highly regarded police official considered to be a rising star in the city’s law-enforcement bureaucracy, died in a motorcycle crash on a portion of Interstate 280 closed for earthquake repairs. He was 40.

Mayor Art Agnos ordered city flags lowered to half-staff in honor of the 19-year Police Department veteran, who was killed April 14, 1990 returning from Candlestick Park. Cmdr. Nelson recently had been commended for his role in the evacuation of Candlestick after the Oct. 17 earthquake.

There was a moment of silence for Cmdr. Nelson at the Giants’ game following his death.

The mayor called him “a brilliant police officer whose professional future had no limits.” Amos’ comments were echoed by Cmdr. Nelson’s fellow officers and others who knew him. The mayor said: “Isiah’s sudden and tragic death has shocked and deeply saddened all of us who knew and respected him.

“He was the embodiment of the new generation of leadership in the Police Department and a model for anyone who wanted to be a police officer. The entire city joins his wife and family in mourning his death.”

According to police reports, Cmdr. Nelson died in a solo crash of his motorcycle at 12:15 am. He was en route to the Hall of Justice from Candlestick Park where he had been on duty in connection with the game between the Giants and San Diego Padres. His bike crashed into a cement barrier on the closed freeway near 25th Street.

Police sources who were at the scene said Cmdr. Nelson apparently went around one of the barriers, which do not span all the lanes, but are set in a maze and require a vehicle to weave through them. After negotiating that barrier, he apparently ran into trouble.

Assistant Police Chief Willis Casey said officers on motorcycles use the closed section of the interstate because they believe it is safer than dodging traffic on Highway 101.

Cmdr. Nelson, who was 36 when he was promoted to the rank of commander, was the youngest officer and the first black to serve in that rank in the department. He wore badge No. 869 and was nicknamed “Ike.”

Sgt. Jerry Senkir, a spokesperson for the department, said he had known Cmdr. Nelson since he was a Police Athletic League cadet at the Northern Station. “This was his career. He prepared himself to move through the ranks. He went to school. . . He was a well-respected and much beloved officer. I never heard anyone say anything bad about Ike.”

Cmdr. Nelson headed the department’s burglary detail before he was promoted to commander of field operations in 1986. In later reorganizations he was given charge of the special operations bureau. At the time of his death, he oversaw the First Patrol Division, which covers Central, Southern, and Potrero stations. For the last two years, he was in charge of security for Giants’ and 49ers’ games.

“Every job we ever gave him, he put his heart into it 100 percent,” Assistant Chief Casey said. “He went through several reorganizations, and each time he got a new job, and each time he did one hell of a job. He had a bright future.” Cmdr. Nelson once told an Examiner reporter that “the ethics my father and mother instilled in me” helped him to excel. “I always try to do what’s right,” he said. “I learned early in this job that no matter what you do you can’t please everyone, so I do what’s best for San Francisco.”

Cmdr. Nelson is survived by his wife, Dorian, and their two children, Gabriel, 8, and Anthony, 11 months.