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.

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

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 Supt. Phil Cline.

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 attorney who is the father of a Chicago police officer. Another was a 66-year-old Chicago patent attorney.

Emergency calls went out, including one from Wood Phillips attorney Dean Monco. After placing the call, Monco walked out of his office, where a man he didn’t recognize pleaded with him not to move, the lawyer told his wife Cristina.

Instead of stopping, Monco bolted for a stairway exit as the assailant shot at him and missed, he told his wife. On his way out, he saw one of the lawfirm’s partners on the ground. He called his wife from a lower floor to say he had made it out.

Other coworkers in the upper floor offices of the law firm hid under desks, frantically calling family members and emergency officials. Downstairs, people hurried down the escalators and ran from the West Loop skyscraper, as dozens of police cars and fire trucks converged in the street outside.

Police sealed off a wide area around the building, as SWAT team members rushed in and ambulances waited outside. The SWAT team members rode a freight elevator to the 37th floor, then sneaked in through a different door to the office.

More than 25 employees on the floor were rescued, Cline said. But officers who responded to the scene described it as “devastating,” and some paramedics became emotional and had to be consoled.

By 3:40 p.m., a man bleeding from his mouth was wheeled out of the building on a gurney. Meanwhile, other officers were trying to negotiate with the gunman as he held a male hostage. Automatic emergency recordings interrupted the process, prompting repeated calls from frustrated officers to shut them off.

At 3:55 p.m., those calls included a request for all available police officers to gather in the building’s lobby.

Upstairs, officers confronted the man in a hallway, where he alternately pointed his gun at the head of his hostage and his own head.

Cline said that when the SWAT officers had a clear shot, they opened fire. It was about a 45-yard shot taken with a sniper rifle, he said, and the bullet fatally wounded the suspect.

Shortly after 4 p.m., officials reported the scene was secure.