Chicago ? A man chained himself inside a downtown law office Friday, fatally shot three men and was holding another person hostage before he was killed by a police sharpshooter.

The assault on an upper floor of the Citigroup Center put the Ogilvie Transportation Center on lockdown below and stranded commuters there for much of the evening rush.

Sources said they believed the shooter was a disgruntled former client of the attorney he had asked to see. The attorney, who leased space in the offices of intellectual property law firm Wood, Phillips, Katz, Clark & Mortimer, was one of the men slain, according to sources.

The gunman also killed a Wood Phillips attorney and another employee of the firm, the sources said.

Police accounts of the killing shifted slightly through the evening Friday.

Late Friday, police said the man drew a gun and ordered a security guard to take him up to the law offices. Earlier, police had said he brought the weapon into the law office secreted inside a manila envelope.

Police said the gunman arrived just before 3:15 p.m., armed with a snub-nosed revolver, a knife and a short-handled sledgehammer.

“Over the next several minutes, he shot four victims,” said police Superintendent Phil Cline.

An unidentified person is taken Friday from a commuter train station in Chicago. A gunman and three other victims were killed after the gunman opened fire Friday afternoon inside a downtown law office.

The gunman held another man hostage for most of the next hour, until police SWAT team members fatally shot the assailant in a tense hostage standoff in which the assailant may also have shot himself, Cline said.

A fourth shooting victim, a 57-year-old woman with a bullet wound to the foot, was treated at Rush University Medical Center. The hostage was not harmed.

After the standoff ended, Metra officials reopened the station at 5 p.m. For more than an hour, throngs of grumbling commuters boarded trains as soon as the doors would open to rush them away.

Cline declined to identify the dead or wounded, but said the man the suspect had come to the office to see was killed.

Police “believe (the shooter) had previous encounters with people in the office,” Cline said.

Sources identified the victims as a 58-year-old patent and trademark lawyer who is the father of a Chicago police officer. Another was a 66-year-old Chicago patent lawyer.