Milwaukee was shaken awake early Wednesday morning by an explosion at gear manufacturing plant Falk Corp., 3001 W. Canal St., that left three dead and injured 46 others with one person in critical condition.

The explosion occurred minutes after 8 a.m. and started a fire that raged for nearly two hours, leaving the building a blackened skeleton of its former self.

Initially, officials said a propane tank exploded, but later took back that statement.

"We don't have a final determination of what happened," said Linda Mayer, vice president of corporate communications for Falk Corp.

According to Milwaukee Police Chief Nannette Hegerty, employees began evacuating the area upon discovering a propane leak in a building on the Falk complex. During evacuation, a huge explosion occurred.

Hegerty said officials do not know what triggered the explosion but that the death toll would have been much higher had employees not already started evacuating.

The explosion could be felt and heard miles away.

"The whole house shook. Things fell off the walls and the refrigerator," said Roxanna Rick, who has lived two miles from the plant at North 34th Street and West Park Hill Avenue for 56 years. "I thought a car hit my house."

Rick said she watched traffic on I-94 slow to a crawl as emergency vehicles tried to reach the scene. Shortly after, police closed both the 27th Street and 35th Street bridges over the freeway.

The force of the explosion knocked two Plexiglas windows out of their frames at the BP gas station behind Rick's house.

Many Marquette students were awakened for morning classes by the explosion.

Jenny Heiser, a sophomore in the College of Arts & Sciences and resident of Schroeder Hall, said she was getting ready for class when the lights in her room flickered on and off several times. Moments later she heard a loud boom.

"I just thought it was thunder," she said.

All 12 of Milwaukee's emergency services – including fire, police, health and public works departments – were dispatched to the scene. The first Milwaukee Fire Department vehicle reached the plant less than four minutes after the initial 911 call.

Rick said she saw smoke and flames in the Menomonee Valley and attempted to call 911 but phone lines were jammed.

Lt. Larry Jenkins of MFD said 37 fire and rescue units, made up of 120 personnel, responded to the scene.

Milwaukee Mayor Tom Barrett applauded the unified emergency efforts at a mid-morning press conference held at Miller Park.

"This is an example of cooperation among agencies," Hegerty said.

Family members of employees also gathered at the stadium for information and counseling.

Mayer said the manufacturing site employs around 700 workers during each of three shifts. The explosion occurred during the plant's first shift of the day and Mayer guessed that around 100 employees were in the building.

"The first thing we did was to make a list and try to account for everyone," Mayer said. All employees were accounted for by 5 p.m. Wednesday.

The Milwaukee Police Department and the district attorney's office began a criminal investigation that Hegerty said she expects to last one week. Police officials have already interviewed 500 Falk employees.

Hegerty stressed that a criminal investigation is standard procedure and no foul play is suspected at this time.

Falk Corp. was established in 1856 as a brewery. By 1899 it had changed to a gear manufacturing company.

"Falk has always been there," Rick said. "I can't believe it. This is too much for our quiet neighborhood."

Terri Sheridan and Tim Horneman of the Tribune staff contributed to this report.