WASHINGTON — Federal transportation safety officials say Metro must immediately repair and install sleeves that are designed to protect electrical components, which power the rail system, from water and other contaminants.

The National Transportation Safety Board issued its recommendation Monday. The NTSB discovered the problem while investigating the January smoke that filled the L’Enfant Plaza Station and killed a rider from Alexandria, Virginia.

Electrical connections that provide power to the high-voltage third rail were “improperly constructed and installed.” If moisture were able to get into the electrical components, the components could short-circuit, generating fire and smoke, according to the NTSB.

Investigators found that sealing sleeves designed to protect electrical connections from water or other contamination were missing while they reviewed a similar smoke event outside the Court House Station in Virginia in February. The sleeves also were missing from other locations in the rail system, the NTSB says.

Electrical arcing created acrid, yellow smoke that filled a Yellow Line train and tunnel on Jan. 12. But investigators still don’t know what caused the arcing. And the electrical components were so badly damaged that investigators can’t tell whether or not the protective sleeves were installed to protect the electrical components outside L’Enfant Plaza.

The NTSB has scheduled a two-day fact-finding hearing to assist in its investigation into the fatal smoke event. The hearing will be held June 23 and June 24 at the board’s main office at L’Enfant Plaza.

The Associated Press contributed to this report.