Witnesses say there was no physical altercation between Tampa bus driver Thomas Dunn and the man who killed him.

Dunn, 46, who had complained about driver safety issues in December, was working his regular route when police say Justin Ryan McGriff, 35, stabbed him and ran away.

Tampa Police spokesman Steve Hegarty announced Sunday that McGriff had been arrested on charges of resisting arrest and first-degree murder.

With no discernible motive to explain McGriff's actions, witnesses, officials and family members were left to sort through a confounding tragedy.

There was much to consider Sunday:

First, the victim's heroics.

Saturday afternoon, the driver steered his Hillsborough Area Regional Transit Authority bus along Nebraska Avenue between Lotus Avenue and Castle Court.

That's when McGriff allegedly stabbed him.

Despite the injuries, the driver was able to pull the bus to the side of the road, police said. None of his passengers were hurt.

Nicaisse Tassy, the owner of Camelle Cuisine Haitian Restaurant, recalls seeing the HART bus stopped in front of his Nebraska Avenue establishment. Police soon surrounded the bus and blocked off the scene with yellow caution tape.

"I had no idea what was going on," said Tassy. "It's when I saw the bus driver slouched over the wheel in his seat, then I knew something terrible has happened."

After the violence came the chase.

Police identified McGriff after interviewing witnesses on the bus. Officers found him near the scene of the crime, and — after a brief pursuit — they arrested him on the nearby shoulder of the northbound lanes of Interstate 275.

Records show McGriff was twice convicted on burglary charges in Oklahoma about a decade ago.

Elmer McGriff, Justin's uncle in Tulsa, Okla, was surprised when a reporter told him of his nephew's arrest.

"That don't sound a doggone thing like him," McGriff, 50, said.

McGriff and his nephew lived together for eight months about five years ago. He said Justin mostly kept to himself during that period — he spent most of his time at his jobs, working in construction and at a barbecue restaurant.

"He's very mild-mannered, kind of reclusive — an introvert," McGriff said.

"You wouldn't know that he was there if it wasn't for his size."

The two fell out of touch, McGriff said, but he last saw Justin about eight months ago at a funeral.

"He really seemed like he had it together," he said.

Now comes the reckoning.

The 200,000-plus-member Amalgamated Transit Union condemned the driver's killing and called for safer conditions for transit workers.

"Transit workers cannot go to their job, and passengers cannot travel on public transportation in fear of being attacked and assaulted," ATU International Executive Vice President Javier Perez said in a statement Sunday.

Last December, Dunn complained about bus driver safety at a Hillsborough Area Regional Transport Authority meeting, after being harassed by a passenger.

"It seems to me admin is taking care of admin and not taking care of the folks at the wheel that actually make this company work," he said.

Dunn's brother, Michael E. Dunn, wrote a tribute praising him on Facebook Sunday afternoon.

"Despite being fatally wounded, Thomas Dunn was able to safely stop his bus. Passenger safety was his first and last priority!" he wrote.

Times senior researcher John Martin and Times staff photographer Octavio Jones contributed reporting.

Contact Kirby Wilson at kwilson@tampabay.com or (727) 893-8793. Follow @kirbywtweets.

Contact Kavitha Surana at ksurana@tampabay.com.

Contact Paul Guzzo at pguzzo@tampabay.com or (813) 226-3394. Follow @pguzzotimes