Speed and following too closely caused a collision that led to multiple pileups on Interstate 5 near Boeing Field on Friday morning. Traffic was backed up for 15 miles, authorities said.

Five people were hospitalized. One of them, a 25-year-old man, was rushed to Harborview Medical Center in critical condition after the chain-reaction crashes in the northbound lanes. He was still critical and in intensive care late Friday afternoon, according to hospital spokeswoman Susan Gregg.

Some 30 people were evaluated by Seattle Fire Department medics, including five who were involved in a five-vehicle collision directly across the roadway in the southbound lanes, said Seattle Fire Department spokesman Kyle Moore.

State Patrol Trooper George Englebright said a motorist traveling in the carpool lane rear-ended a vehicle, which triggered other collisions in the carpool lane and nearby lanes. The State Patrol is deciding whether to issue citations for speeding and following too closely to the driver who caused the initial carpool-lane crash.

Between about 6:45-8 a.m., all lanes of northbound I-5 were shut down while authorities investigated the crashes and treated the injured.

The 25-year-old man who was critically hurt was a passenger in a car that went off the roadway and into some blackberry bushes. He suffered a head injury, Moore said.

Jennifer Sullivan: 206-464-8294 or jensullivan@seattletimes.com. On Twitter @SeattleSullivan.