Robert Massaroni had been watching the masters teach martial arts since he was a child.

Finally, when he was around 10, he talked his mother into enrolling him as a student at the Black Belt Academy in Bensalem. He worked his way up to fourth-degree black belt in tang soo do, a Korean martial art, and had just tested for his master's belt, a fellow instructor said yesterday.

Angela Pote said she, master Louis Castelli - one of Massaroni's first teachers - and other instructors were in shock after learning that Massaroni, 29 and physically fit, had collapsed and died Sunday after crossing the finish line at the Broad Street Run.

The medical examiner's office has not yet released a cause of death, Jeff Moran, spokesman for the Philadelphia Health Department, said yesterday.

Massaroni, of Holland, Bucks County, a seventh-grade social- studies teacher at the Robert K. Shafer Middle School in the Bensalem School District, finished 3,263rd, with a time of 1 hour, 21 minutes and 34 seconds in the 10-mile run.

He also completed the run last year.

"You would never expect it," Pote said of Massaroni's death. "There was no indication that there was anything [wrong]. All those things are a complete mystery to us right now."

Franco Propato, brother of Massaroni's wife, Val, said Massaroni had been running for years.

He described his brother-in-law as "the type of person you never saw without a smile - just a genuine person, nothing fake about him."

Others filled martial-arts master Nate Gordon's Web page with tributes to the man known by his students as "Mr. Mass."

Massaroni was also a teacher of martial arts. His youngest students, at age 4, were known as the "Little Dragons."

Pote said Massaroni had participated in the Broad Street Run two or three times before.

"He was looking forward to the race," she said. "He definitely was training up to it."

Tang soo do is a "kicking art, mainly, an active art," Pote said. "It's not one of those calm martial arts like tai chi or anything. It's very fast-moving.

"You definitely have to be in shape to do this."

She said Massaroni was a dynamo. "He just had a good spark about him," Pote said. "His energy, his personality, was just amazing. He was one of those people who came early and left late."

Students from his middle school added their farewells to the Massaroni tribute site.

"I will miss the way you smiled," one student wrote. "I will miss your laugh.

"I will even miss the way you used to make me get to class. I will even miss the way you yelled at me to be quiet.

"I can't believe I'm saying this but, yes, I will even miss the way you made me throw away my gum."

Staff writer Christine Olley contributed to this report.