Eintracht Frankfurt fired four goals past RB Leipzig on Saturday to climb up the Bundesliga table and send their opponents deeper into crisis ahead of their Champions League group stage openers.
The Bavarians put down a marker at the start of the campaign as they look to land an 11th consecutive league crown.
Eintracht Frankfurt won the Europa League final after holding their nerve to beat Rangers 5-4 in a clinical penalty shootout as they claimed their first European trophy in 42 years.
West Ham said they were aware of the incident and would work to identify the offender.
Frankfurt go through to the last four and the German club will now be eyeing a first European triumph since they won the UEFA Cup in 1980.
Real Madrid have signed Serbia striker Luka Jovic from Eintracht Frankfurt for 60 million euros (£52.39 million), television channel Sky Germany said on Thursday.
Jovic may have grabbed the headlines and turned the heads of Europe's top clubs with his goals this season, but he is just one of a trio of brilliant Balkans.
Luka Jovic has been hailed as a "potential world-class striker" after the 20-year-old's five goals in Eintracht Frankfurt's 7-1 drubbing of Fortuna Dusseldorf.
Borussia Dortmund squandered a two-goal lead to draw 2-2 at Eintracht Frankfurt on Saturday.
Eintracht Frankfurt on Friday signed midfielder Kevin-Prince Boateng on a two-year deal with the former Ghana international returning to the Bundesliga after quitting Spain's Las Palmas.
Eintracht Frankfurt announced the signing of the first female scout in the Bundesliga on June 18, Portuguese-born Helena Margarida dos Santos e Costa.