He struck one bar patron in the head with a glass and then began swinging a bar stool, according to D.C. police. One customer tried to subdue the unruly man early Friday, and during the tussle, both tumbled down the stairs of an Adams Morgan tavern, authorities said.

The man who stepped in to help — a 45-year-old former middle school teacher and band bass player — was in critical condition Friday at George Washington University Hospital. A police report said he suffered a fractured skull and a broken neck and was hemorrhaging from the brain.

The alleged instigator of the fracas was in the same hospital, also in critical condition with a fractured skull. D.C. police identified him as James Wood, 37, who lives a block from the bar, and said they charged him with assault with a dangerous weapon.

Authorities said detectives are still investigating the incident.

Friday night, Police Chief Cathy L. Lanier ordered the bar — Rendezvous Lounge in the 2200 block of 18th Street NW — to be closed for four days, deeming it a public-safety hazard, a spokeswoman said. The chief has taken a hard line on violence at liquor establishments: She temporarily revoked the license of the Reef, another Adams Morgan bar, after a stabbing there this month.

Adams Morgan has the highest bar-per-block density in the District and is often packed on weekends. It’s one of the city’s original nightlife districts, and a platoon of police is permanently assigned there in the summer during the night and early morning.

The owners of Rendezvous could not be reached to comment.

Friday’s altercation started about 1:50 a.m., after last call and minutes before closing time. The man who was hit with the glass said in an interview that he knew neither the assailant nor the man who intervened.

He said the attack on him was unprovoked. “I just got hit in the head with something,” said the 38-year-old man, who spoke on the condition of anonymity. He said his attacker came into the bar just as it was closing and immediately made a scene. The few customers remaining were getting ready to leave, he said.

Police said that after the glass was thrown, the man picked up the stool and swung it widely but did not hit anyone. The witness said that after several people wrestled the stool away, the man “was trying to slug everybody.”

Police said that Wood and the 45-year-old patron then grappled near the stairs, and both went down.

Court records show that Wood has had several run-ins with the law in the District and in Montgomery County. He has been convicted of robbery — in connection with a holdup of a Wheaton bank in 2007 — and of assault, in 2006 in Montgomery. Last year in the District, he was charged with threatening a cabdriver, but the case was dismissed. His mother, when reached by phone, declined to comment.

In the robbery case, his attorney initially argued that Wood was not criminally responsible by reason of insanity but withdrew that assertion. Police said he stole $2,700 from the bank after he walked in wearing a suit and a blue ski mask and holding one hand in a brown paper bag. After he left, police said, he shed his suit and put on a sweater swiped from the employee lounge.

Wood pleaded guilty to robbery and was sentenced to 10 years in prison, with all but 18 months suspended.

Court records show that upon his release, he “could not live” at his family’s home in Chevy Chase, so he moved to Adams Morgan.

The mother and sister of the critically injured patron, who live in Colorado, were en route to the District on Friday and could not be reached to comment. A cousin confirmed that the man had been a teacher; a D.C. school system spokeswoman said he had been employed through 2010.

The cousin also confirmed that the victim is a member of a local band, led by a singer from Moscow, that plays a mix of punk and old rock songs.

According to the band’s Web site, the victim started playing country-western music in the Midwest. He plays bass, the site says, “and his bass lines are sure to keep people boogying down all night!”

Emma Brown, Jennifer Jenkins and Dan Morse contributed to this report.