PORTSMOUTH, N.H. – First came an explosion on a 600-foot container ship. Then came word that a tugboat had been hijacked and another vessel pinned against a bridge was on fire.

The events were fictional. The response was real.

Frontier Sentinel, a five-day exercise to test the skills, communication abilities and equipment of about 3,000 sailors and other homeland defenders from the U.S. and Canada, is taking place this week off the Maine and New Hampshire coast.

“The whole purpose is to prepare for all hazards and all threats,” Coast Guard sector commander Jim McPherson in Northern New England said Tuesday as those involved showed their stuff for the news media.

The drill kicked off about 9 a.m. Monday with news of the shipboard explosion. The Coast Guard and Navy, with help from the FBI and local law enforcement, investigated. They pretended to close the port.

When they discovered that submerged mines may have been involved, Canada’s Joint Task Force Atlantic stepped in with a seven-ton, remote-controlled mine detector. Called the Dorado, the device can be controlled from about three to five miles away.

Six other unmanned, undersea vehicles were loaded into the ocean from docks and vessels. Twenty-four American and Canadian divers also searched the ocean floor for the fictional mines. The vehicles transmitted images to Coast Guard Station Portsmouth Harbor, where several U.S. Navy experts sat in front of computer screens examining them.

They looked at the size of shadows, smoothness and other characteristics that could indicate a mine rather than objects ranging from lobster pots to rocks.

Once probable mines were identified, the task became deciding how to eliminate them.

“The purpose of the exercise is not really the mines,” McPherson said. “It easily could have been a hurricane that closed down the port because a bridge collapsed. The overarching theme is to test our ability to communicate and to coordinate actions at a federal, state and local level.”

Officials declined to give details of the fictional incidents and the response because the exercise was continuing.

The area was chosen because of its proximity to the Portsmouth Naval Shipyard and other industrial facilities, fast tidal currents, limited underwater visibility and complex ocean floor.

About 600 of those involved are at or near the shipyard, which is in Kittery, Maine, or the Coast Guard station a short boat ride away in Portsmouth.