Authorities investigating the collapse of the Interstate 35W bridge are looking for a person who was in a kayak below or near the bridge when it tumbled into the Mississippi River last week, Minneapolis Police Capt. Mike Martin said Wednesday.

"We need to talk to that person," Martin said.

Speaking at a press conference Wednesday, one week after the collapse of the bridge during rush hour killed at least five people and injured 100, Martin also warned area residents to stay clear of the disaster site, where perimeter security has become a top concern.

Thousands of people continue to flock to the site, and 14 people have been arrested for breaching the perimeter so far. That rate has slowed with just two arrests Tuesday, Martin said.

Additional security measures, such as cameras and motion detectors, were being installed at the site to assist authorities in identifying people who were crossing the perimeter, he added.

"You're not going to get anywhere near it without being arrested," Martin said.

Following an official day of mourning Tuesday, flags flew at half staff Wednesday in the Capitol in St. Paul — the other half of the Twin Cities — as Navy, FBI and local dive teams continued the search for eight people who remain missing and are presumed dead among the wreckage.

"It is a very dangerous, very tenuous situation down there. The current is constantly changing, debris is shifting," Martin said. "It is a slow, methodical process."

Progress in actually clearing the debris from the water would be slow, he said.

"These vehicles had debris fall on them. They are not easily removable," Martin said. "We won't be lifting heavy parts out any time soon."

After working until 10 p.m. Tuesday, Navy divers were back at the river at 7 a.m. Wednesday and were doing "a very meticulous, hand-over-hand search of the scene," said their spokesman, Senior Chief Dave Nagle.

The FBI team had to abandon the use of the larger of their two unmanned submarines, Special Agent Paul McCabe said Wednesday. The remote-controlled vehicle — equipped with a camera, sonar, lights and a grabbing arm — was too big to maneuver amid the unstable, twisted bridge wreckage and vehicles in the murky water, he said.

Instead, FBI divers will use their smaller sub, a shoe box-sized vehicle equipped only with lights and a camera. It also has smaller thrusters which make it more susceptible to the stiff river currents.

There was no change overnight in the condition of the only victim of the bridge collapse still in critical condition, said a spokeswoman for the Hennepin County Medical Center. At least seven other victims remained hospitalized.

Church bells tolled in downtown Minneapolis Tuesday as residents observed a moment of silence for the victims. At 6:05 p.m. — the time of the collapse — crowds gathered at two parks near the broken span and along an upstream bridge, removing their hats and bowing their heads.

At one observance, people threw flowers into the river and poured a vial of water into the river after blessing it.

"This is how we can really reverence the silence of the dead," said Sister Rita McDonald of the Sisters of St. Joseph.

One of the heroes of the collapse, Jeremy Hernandez, got good news Tuesday. Hernandez, who dropped out of Dunwoody College of Technology when he couldn't pay the tuition, was hailed for helping evacuate a school bus full of children. The school is now offering him the chance to finish his two-year automotive technician degree for free.

State officials have announced tentative plans for a replacement bridge, with five lanes each way instead of four. The new bridge also might be built to accommodate future bus rapid transit or light rail service.

Officials said they will start narrowing the field of potential contractors this week and by Sept. 1 they hope to select the builder. The deal will include incentives for early completion.

Gov. Tim Pawlenty said the aggressive timeline — the goal is to have it open before the end of next year — won't mean corners are cut.

"We are going to get this bridge built safely, number one," he said at a news conference. "So we're not going to go so fast or emphasize speed that the bridge isn't done well or done correctly."

Authorities are urging anyone who was on or near the bridge at the time of the collapse and has not yet been formally interviewed by investigators to call the National Transportation Safety Board hotline, 1-866-328-6347.