Traffic is seen being diverted off I-43 southbound at Fond du Lac Ave.

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Four people have been arrested and others may still be on the loose after a shootout Wednesday morning on I-43 that injured a man and prompted authorities to shut down the southbound lanes for several hours.

Southbound I-43 through downtown Milwaukee from W. Fond du Lac Ave. to W. Becher St. was closed while investigators walked down the highway to recover shell casings and bullet fragments, the Milwaukee County Sheriff's Office said.

The highway lanes reopened about 2:30 p.m. Wednesday.

The sprawling crime investigation began when deputies responded to reports of a shooting on the freeway in the Marquette Interchange near Highland Ave. about 10:10 a.m., according to the Sheriff's Office.

Investigators determined that more than a dozen shots were fired into a vehicle during a gunfire exchange with another vehicle.

A person in one vehicle was grazed by a bullet.

Milwaukee County Sheriff's Office shared this photo of car involved in I-43 shootout. MORE: https://t.co/0DXhsVSjWc pic.twitter.com/VbOvL8TDRO — Journal Sentinel (@journalsentinel) December 9, 2015

In the ensuing investigation, the Sheriff's Office said it responded to four separate scenes regarding four vehicles that may have been involved in the incident.

All vehicles were towed as evidence.

As of Wednesday afternoon, four people were in custody.

The person grazed by a bullet stopped the rental vehicle he was driving near S. 10th and W. Walker streets, according to the release. Deputies recovered a weapon and arrested the driver and a male passenger.

Milwaukee police conducted a traffic stop around 10:20 a.m. at N. 17th and W. State streets involving a stolen vehicle with bullet damage. Two of the four people in the vehicle were arrested after a foot chase, police said just before 1 p.m., but they were still seeking the two suspects who escaped.

Another stolen vehicle was found abandoned in the Marquette Interchange with drugs inside, according to the Sheriff's Office.

The driver fled on foot, and it wasn't clear whether the person was involved in the shootout.

A fourth vehicle, which had a tire blown out by a bullet, exited at Plankinton Ave., according to the Sheriff's Office, which didn't elaborate on the status of that driver.

On scene 17th & State @MilwaukeePolice got 2 & looking for 2 suspects who ran after car chase. Keep it on @fox6now pic.twitter.com/AK3r3MzMag — Katrina Cravy (@KatrinaCravy) December 9, 2015

No further information will be released while the investigation continues, the Sheriff's Office said.

Anyone who saw the incident is asked to call the county dispatch center at (414) 278-4788.

This is at least the third time in recent years that shots have been fired on I-43.

In 2013, a shootout occurred on the highway involving a pair of men with concealed-carry permits.

And in 2014, a crash involving two vehicles near Dean Road escalated when a person in one of the vehicles fired several shots at another vehicle from a short distance.