A cement truck collided with two other vehicles and crashed through a jersey wall on the Capital Beltway in Northern Virginia today, shutting down all lanes on the inner loop for more than three hours and causing massive traffic jams, police said. Three people were reported injured.

Police reopened all lanes on the inner loop of I-495 at the scene of the accident shortly before 2:30 p.m.

The accident, in which seven vehicles were damaged, caused massive backups on the Beltway near Route 7.

The crash occurred shortly after 11 a.m. when the cement truck, a pickup truck and a minivan were involved in a collision while traveling on I-495 just north of Route 7, according to Sgt. Terry Licklider, a spokesman for the Virginia State Police. He said investigators were trying to determine the cause of the crash and who might have been at fault.

The cement truck hurtled into the jersey wall separating the northbound lanes from the southbound lanes of the Beltway, then overturned, spilling cement into the southbound lanes of the outer loop, Licklider said. The collision also sent chunks of concrete from the jersey wall into the oncoming southbound traffic, damaging four vehicles, he said.

The driver of the cement truck was taken to a hospital with apparently serious internal injuries, Licklider said. Two people from the minivan were also taken to the hospital for treatment. No other injuries were reported.

About 50 gallons of diesel fuel were spilled on the roadway as a result of the accident. The Fairfax County Fire Department cleaned up the spill with absorbent material and used water hoses to wash the concrete off the highway, Licklider said.

By 12:30 p.m., traffic was backed up at least five miles in the northbound lanes and a couple of miles in the southbound lanes.

With all lanes blocked on the inner loop, traffic was diverted to a service road to get by the accident scene.

In a separate accident in Maryland, a car plunged off the Clara Barton Parkway about 11:20 a.m. and hung precariously in a vertical position, trapping a woman inside, a spokesman for the Maryland State Police in Rockville said.

Pete Piringer, spokesman for Montgomery County Fire and Rescue, said the woman was extricated from the car at about 11:40 a.m. with only minor injuries.

The incident occurred on the ramp from the Parkway to the outer loop of the Beltway in Maryland. Traffic was reported to be squeezing by in a single lane along the ramp.

Piringer said the car apparently was on the ramp leading from the Clara Barton Parkway to I-495 South when it left the roadway, went airborne and plunged over a guardrail into some trees, where the vehicle was perched in a vertical position.

Rescue workers managed to stabilize the car quickly. A paramedic got into the car with the driver and managed to pull her out. Piringer said that while the road was wet, no cause for the incident had been determined.