Federal safety investigators have called on the D.C. government to upgrade firefighting and emergency communications equipment in the "Center Leg" tunnel, the six-lane artery that carries about 85,000 vehicles daily under the Mall at 3rd Street NW.

Due to a lack of basic safety equipment, "a simple automobile engine fire could escalate into a catastrophe if it were to spread to other trucks or vehicles," National Transportation Safety Board chairman James Burnett said in a letter to city transportation director Thomas Downs.

Burnett said a board team "noted the absence of fire extinguishers, sand buckets, hoses and fire alarms" during an inspection of the tunnel, which funnels traffic to and from the Southwest Freeway. They also found that emergency telephones were broken and that there was no monitoring of traffic by guards or closed-circuit television.

City officials said they had requested the board's study and had already made some of the improvements recommended.

The board said the tunnel, part of Interstate 395, was inspected as part of its investigation of a fiery accident last April in the Caldecott Tunnel in Oakland, Calif. Seven people were killed in that accident, which involved a gasoline tanker, bus and automobile.

In his letter, Burnett noted that the Center Leg tunnel is off limits to vehicles carrying flammable and other hazardous materials, which he said reduces the chances of a catastrophic accident occurring in it.

City crews have failed to keep fire extinguishers in the tunnel properly charged in the past, Downs said, adding that that would be corrected.

The tunnel originally had the latest in TV monitoring equipment, he said. But over the years, the expensive equipment fell into disrepair. Downs said city highways officials will look closely to see whether current traffic levels warrant restoring use of such sophisticated gear, as the board suggested might be suitable.

D.C. traffic engineering head Seward Cross, meanwhile, said that in the last 30 days, technicians had fixed all of the broken telephones, about one dozen.

Burnett's letter said that additional phones and firefighting gear might also be warranted in the Dupont Circle tunnel, the 9th and 12th Street tunnels under the Mall and an extension of the Center Leg tunnel that is scheduled to open late in 1984 to carry I-395 through to New York Avenue.