(CNN) -- The bright green suits of scuba divers on board a speedboat pierced the dusk, as they perused a mangle of steel girders of a section of bridge that lay collapsed in the waters of a Washington state river late Thursday.
Three people who plunged with their vehicles, a truck and an SUV, into the Skagit River along with the truss structure have been rescued, state authorities said.
"At this point, we don't have indication anyone else went into the water," said incident management spokesman Marcus Deyerin.
Still, a speedboat with the word "Sheriff" beaming in large, white letters from its side, panned from side to side showing the divers various perspectives of the half-submerged structure, as they prepared to splash into the current.
They want to double-check that no one was trapped under the surface, Deyerin said.
A helicopter hovered over them, and other rescue boats skimmed the waters nearby.
Old bridge, possible accident
What made the section give way is under investigation, but a traffic mishap may have weakened the structure.
"An 18-wheeler commercial motor vehicle with a very large container appears to have struck part of the bridge at some point," said Washington State Trooper Mark Francis. "We cannot say at this time if this was prior to the collapse or is the cause of the bridge collapse."
The bridge on Interstate 5 near Mount Vernon was first constructed in 1955, according to CNN affiliate KOMO.
Skagit County, which lies between Seattle and Vancouver, Canada, lies on the Pacific coast and includes many islands. Of its 223 bridges, the Federal Highway Administration termed 11 "structurally deficient," meaning that they need repairs. Fifty-five were listed as "functionally obsolete" in the 2009 listing. The term indicates that bridge's design is from another era. Its features quaint by modern standards.
The number of bridges the state's transportation department considers "structurally deficient" has grown nearly 50% in the last six years, according to a 2012 government report. At the time 11% of the "deck area of all bridges" in Washington were in the category.
Some 77,000 vehicles pass through the overhead iron framework of the I-5 bridge daily, a spokesman for the state's transportation department said, and DOT engineers are investigating if an oversized load may have overburdened the structure before it gave way.
The bridge crosses the Skagit River in a rural area, Francis said.
CNN affiliate King 5 News in Seattle broadcast images of the evening collapse, showing a span of the bridge in the water and crowds nearby.
Images included two people sitting atop their vehicles in the water.