Howes, who plays for U.S. head coach Keith Tozer with the Milwaukee Wave of the Major Indoor Soccer League, scored in the 26th minute and Philadelphia Kixx star Shanker tallied from Wave star Todd Dusosky's assist in the 31st minute of the Group B contest.
Gilberto Rangel scored twice for Panama, netting the decisive goal a minute after Shanker's strike.
''We felt that we came into this match well-organized,'' Tozer said. ''We created a lot of goal scoring chances, but we just did not finish.''
The U.S. team meets Guyana on Monday and Cuba on Wednesday. The winners of Friday's semifinals qualify for the FIFA World Futsal Championship to be played Nov. 21-Dec. 5 in Chinese Taipei.
Midfielder John Ball, of the Cleveland Force, will miss the game against Guyana after receiving two yellow cards in the opener. Ball also plays for the Atlanta Silverbacks of the A-League.
Cuba won its Group B opener, routing Guyana, 10-3. Costa Rica and Mexico won Group A openers on Friday.
Futsal is a cousin of North American indoor soccer, featuring five-player teams on a basketball-like court with no walls and a low-bouncing ball. Indoor soccer features six-player teams and side- and backwalls on a hockey rink-sized field.
CONCACAF QUALIFYING TOURNAMENT
July 23-31 at Heredia, Costa Rica
U.S. Schedule
July
24: United States 2 Panama 2
July 26: vs. Guyana
July 28: vs. Cuba
July 30: Semifinals
July 31: Final/third place
Group A
W L T
GF GA Pts
Costa Rica 1 0 0 13 1 3
Mexico 1 0 0 6 4 3
Trinidad & Tobago 0
1 0 4 6 0
Suriname 0 1 0 1 13 0
Friday, July 23
Mexico 6 Trinidad & Tobago 4
Costa Rica 13
Suriname 1
Sunday, July 25
Mexico vs. Suriname
Costa Rica vs. Trinidad & Tobago
Tuesday, July 27
Costa Rica vs. Mexico
Trinidad & Tobago vs. Suriname
Group
B
W L T GF GA Pts
Cuba 1 0 0 10 3 3
United States 0 0
1 2 2 1
Panama 0 0 1 2 2 1
Guyana 0 1 0 3 10 0
Saturday, July 24
United States 2 Panama 2
Cuba 10 Guyana 3
Monday, July 26
United States vs. Guyana
Cuba vs. Panama
Wednesday, July 28
United States vs. Cuba
Panama vs.
Guyana
Semifinals
Friday, July 30
Group A winner vs. Group B second
Group B winner vs. Group A second
Third Place
Saturday, July 31
Semifinal
losers
Championship
Saturday, July 31
Semifinals winners
Note: Semifinal winners advance to Nov. 21-Dec. 5 FIFA Futsal World Championship in Chinese Taipei
U.S.
ROSTER
Goalkeepers
Scott Hileman (Baltimore Blast)
Brett Phillips (St. Louis Steamers)
Defenders
Sean Bowers (San Diego Sockers)
Lee Tschantret (Baltimore
Blast)
Pat White (Milwaukee Wave)
Midfielders
John Ball (Cleveland Force)
Danny Kelly (Baltimore Blast)
Joel Shanker (Philadelphia Kixx)
Forwards
Don
D'Ambra (Philadelphia Kixx)
Todd Dusosky (Milwaukee Wave)
Andy Guastaferro (Chicago Storm)
Greg Howes (Milwaukee Wave)
Head coach: Keith Tozer (Milwaukee Wave)
Assistant coach: Vilmar Marques (Foothill College/Los Altos, Calif., and World United FC/Mountain View, Calif.)