Dozens of nonessential personnel were evacuated from Terminal 18, on Harbor Island south of downtown Seattle, a Port of Seattle spokesman said. A bomb squad used explosive charges to cut into the containers, then searched the contents.

It wasn't immediately clear what prompted the dog to alert on the containers. Using detection devices, the Port of Seattle bomb squad and Seattle Fire Department hazardous materials team found no indications of explosives or radioactive materials, officials said.

By early evening, the bomb squad and hazmat team had left, there was no sign of contraband in the containers and the port was preparing to resume normal operations, port spokesman David Schaefer said.

The ship that brought the containers, the MV Rotterdam, is owned by China Shipping, and apparently is the same MV Rotterdam on which 22 Chinese stowaways hid in a container that arrived at Terminal 18 in April. Security guards caught the 22 roaming around the terminal.

Mike Milne, a spokesman for U.S. Customs and Border Protection, said agents used a "gamma-ray" device to peer through the containers' steel walls to determine what they contained. Some of the items did not appear to match what was listed on the containers' manifest, Milne said.

That isn't uncommon, Milne said, and the containers were then checked by the bomb-sniffing dog, per standard procedure. The dog reacted, and agents tested for hints of radiation before opening the containers.

The Port of Seattle bomb squad apparently did not believe there was a chance of causing a larger blast by using the small charges to gain access to the containers, said spokesman Mick Shultz.

"They wouldn't have done it if they thought there was a chance of that," Shultz said.

Shultz said the containers were supposed to contain oily rags, which are often shipped internationally for recycling or use in packaging, but other items were found. He didn't specify what those were.

Nearby businesses were advised to keep their workers indoors during the scare.

Milne said the ship had originated in Hong Kong and made stops in China and Korea before arriving in Seattle on Monday. He said the containers in question were from Pakistan.

Terminal 18 covers nearly 200 acres, making it the port's largest container terminal and one of the largest in the nation. It serves more than 20 steamship lines and receives more than 40 vessels each month.

About 70 members of the International Longshore and Warehouse Union - workers who operate loading and unloading machinery and control the flow of containers - were evacuated just before lunch, as were several dozen truck drivers, said Herald Ugles, president of ILWU Local 19.

A spokeswoman for SSA Marine, which operates the terminal, said the company had no comment.

Sens. Maria Cantwell and Patty Murray, both D-Wash., and Rep. Ed Markey, D-Mass., who is a senior member of the House Homeland Security Committee, said the incident was further evidence that port security is a serious concern.

Markey said containers should be screened before they arrive in the U.S.

"If this false alarm were a real nuclear bomb hidden in a container, federal officials wouldn't have had the chance to evacuate the terminal, let alone all of Seattle," Markey said.