The Navy’s USS John S. McCain, a guided-missile destroyer, collided with an oil tanker east of Singapore in August 2017, the fourth such accident since the beginning of 2017.

Here are the other crashes:

USS Fitzgerald

Seven sailors died when the destroyer collided with a container ship off the coast of Japan on June 17, 2017.

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USS Lake Champlain

The ship hit a South Korean fishing boat near the Korean Peninsula on May 9, 2017.

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USS Antietam

The ship ran aground off the coast of Japan on January 31, 2017, and discharged up to 1,100 gallons of hydraulic oil.

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Lawmakers and defense analysts are reviewing the incidents to see if the crashes are part of a systemic issue. They point to concerns including:

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1. Budget

Maintenance and training dollars are often the first to be cut and the Pentagon has faced constrained budgets under the 2011 Budget Control Act.

Mass Communication Specialist 2nd Class Joshua Fulton/U.S. Navy via Getty Images

2. Fleet size

The Navy has attempted to compensate for a smaller fleet by using the ships in more innovative ways. More ships are working together instead of singularly.

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3. Low light & high traffic

Some of the most difficult times to operate a ship are at sunset and sunrise. Maneuvering also becomes more difficult when the water is crowded with other vessels.

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4. The Pacific

The fact that all four Navy collisions happened in the Pacific could point to issues with training that are specific to the region.

Mass Communication Specialist 3rd Class Adam K. Thomas/U.S. Navy via Getty Images