WASHINGTON (AP) -- Only essential navy employees were back to work Wednesday at the Washington Navy Yard.

Beginning at 9:00 a.m., many workers were allowed to remove their vehicles left in parking lots and garages. They had to eave them in the wake of Monday's shootings.

Investigators also were back at the base to continue to map the crime scene.

Monday's shootings that left 13 people dead, including gunmanAaron Alexis, has led to at least three investigations into security and background screenings.

The secretary of the Navy is ordering a pair of security reviews -- focusing on how well the Navy protects its bases, and how accurately it screens its workers. That's because Alexis had a valid security pass despite erratic behavior and mental health problems.

At the White House, President Barack Obama has ordered his budget office to examine security standards for government contractors and employees across federal agencies.

Officials say Aaron Alexis had been hearing voices and had been undergoing treatment for mental problems including paranoia in recent weeks, but he still kept his security clearance. The Navy says Alexis obtained his security clearance when he enlisted in the Navy reserves in 2007, which was good for 10 years and remained valid after he left the service in 2011. During his time of active service, the Navy says Alexis had incidents of insubordination, disorderly conduct and being absent from work without authorization, but nothing that gave commanders any clue he was capable of such violence.

An attorney for the Virginia gun store says Alexis bought a shotgun and ammunition there two days before the rampage. Michael Slocum said in an email that Alexis visited Sharpshooters Small Arms Range on Saturday. He says Alexis rented a rifle, bought bullets and used the range. He then bought a shotgun and 24 shells. Authorities say he had a shotgun with him when he entered a building at the Navy Yard on Monday and started firing, killing 12 people.

Slocum says the store ran a federal background check on Alexis and it was approved. Law enforcement visited the range Monday, reviewing video and store records.

The store is located about 15 miles from the Navy Yard.

The Navy Warfighter and Family Support Center at Joint Base Anacostia-Bolling has established a phone line to provide help to those affected by the tragedy at the Washington Navy Yard. The number is 1-855-677-1755. Clinical Counselors are available for anyone who is seeking assistance.

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